Endocytosis-dependent desensitization and protein synthesis-dependent resensitization in retinal growth cone adaptation

The Axonal Actin Filament Cytoskeleton: Structure, Function, and Relevance to Injury and Degeneration

Early investigations of the neuronal actin filament cytoskeleton gave rise to the notion that, although growth cones exhibit high levels of actin filaments, the axon shaft exhibits low levels of actin filaments. With the development of new tools and imaging techniques, the axonal actin filament cytoskeleton has undergone a renaissance and is now an active field of research. This article reviews the current state of knowledge about the actin cytoskeleton of the axon shaft. The best understood forms of actin filament organization along axons are axonal actin patches and a submembranous system of rings that endow the axon with protrusive competency and structural integrity, respectively. Additional forms of actin filament organization along the axon have also been described and their roles are being elucidated. Extracellular signals regulate the axonal actin filament cytoskeleton and our understanding of the signaling mechanisms involved is being elaborated. Finally, recent years have seen advances in our perspective on how the axonal actin cytoskeleton is impacted by, and contributes to, axon injury and degeneration. The work to date has opened new venues and future research will undoubtedly continue to provide a richer understanding of the axonal actin filament cytoskeleton.

Stochastic growth and selective stabilization generate stereotyped dendritic arbors

Stereotyped dendritic arbors are shaped by dynamic and stochastic growth during neuronal development. It remains unclear how guidance receptors and ligands coordinate branch dynamic growth, retraction, and stabilization to specify dendritic arbors. We previously showed that extracellular ligand SAX-7/LICAM dictates the shape of the PVD sensory neuron via binding to the dendritic guidance receptor DMA-1, a single transmembrane adhesion molecule. Here, we perform structure-function analyses of DMA-1 and unexpectedly find that robust, stochastic dendritic growth does not require ligand-binding. Instead, ligand-binding inhibits growth, prevents retraction, and specifies arbor shape. Furthermore, we demonstrate that dendritic growth requires a pool of ligand-free DMA-1, which is maintained by receptor endocytosis and reinsertion to the plasma membrane via recycling endosomes. Mutants defective of DMA-1 endocytosis show severely truncated dendritic arbors. We present a model in which ligand-free guidance receptor mediates intrinsic, stochastic dendritic growth, while extracellular ligands instruct dendrite shape by inhibiting growth.

Receptor-Ribosome Coupling: A Link Between Extrinsic Signals and mRNA Translation in Neuronal Compartments

Axons receive extracellular signals that help to guide growth and synapse formation during development and to maintain neuronal function and survival during maturity. These signals relay information via cell surface receptors that can initiate local intracellular signaling at the site of binding, including local messenger RNA (mRNA) translation. Direct coupling of translational machinery to receptors provides an attractive way to activate this local mRNA translation and change the local proteome with high spatiotemporal resolution. Here, we first discuss the increasing evidence that different external stimuli trigger translation of specific subsets of mRNAs in axons via receptors and thus play a prominent role in various processes in both developing and mature neurons. We then discuss the receptor-mediated molecular mechanisms that regulate local mRNA translational with a focus on direct receptor-ribosome coupling. We advance the idea that receptor-ribosome coupling provides several advantages over other translational regulation mechanisms and is a common mechanism in cell communication.

Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Failure to Upregulate the RNA Binding Protein ZBP After Injury Leads to Impaired Regeneration in a Rodent Model of Diabetic Peripheral Neuropathy

Most diabetes patients eventually suffer from peripheral nerve degeneration. Unfortunately, there is no treatment for the condition and its mechanisms are not well understood. There is, however, an emerging consensus that the inability of peripheral nerves to regenerate normally after injury contributes to the pathophysiology. We have previously shown that regeneration of peripheral axons requires local axonal translation of a pool of axonal mRNAs and that the levels and members of this axonal mRNA pool are altered in response to injury. Here, we show that following sciatic nerve injury in a streptozotocin rodent model of type I diabetes, this mobilization of RNAs into the injured axons is attenuated and correlates with decreased axonal regeneration. This failure of axonal RNA localization results from decreased levels of the RNA binding protein ZBP1. Over-expression of ZBP1 rescues the in vitro growth defect in injured dorsal root ganglion neurons from diabetic rodents. These results provide evidence that decreased neuronal responsiveness to injury in diabetes is due to a decreased ability to alter the pool of axonal mRNAs available for local translation, and may open new therapeutic opportunities for diabetic peripheral neuropathy.

Flg22-induced Ca2+ increases undergo desensitization and resensitization

The flagellin epitope flg22, a pathogen-associated molecular pattern (PAMP), binds to the receptor-like kinase FLAGELLIN SENSING2 (FLS2), and triggers Ca²⁺ influx across the plasma membrane (PM). The flg22-induced increases in cytosolic Ca²⁺ concentration ([Ca²⁺ ]_i ) (FICA) play a crucial role in plant innate immunity. It's well established that the receptor FLS2 and reactive oxygen species (ROS) burst undergo sensitivity adaptation after flg22 stimulation, referred to as desensitization and resensitization, to prevent over responses to pathogens. However, whether FICA also mount adaptation mechanisms to ensure appropriate and efficient responses against pathogens remains poorly understood. Here, we analysed systematically [Ca²⁺ ]_i increases upon two successive flg22 treatments, recorded and characterized rapid desensitization but slow resensitization of FICA in Arabidopsis thaliana. Pharmacological analyses showed that the rapid desensitization might be synergistically regulated by ligand-induced FLS2 endocytosis as well as the PM depolarization. The resensitization of FICA might require de novo FLS2 protein synthesis. FICA resensitization appeared significantly slower than FLS2 protein recovery, suggesting additional regulatory mechanisms of other components, such as flg22-related Ca²⁺ permeable channels. Taken together, we have carefully defined the FICA sensitivity adaptation, which will facilitate further molecular and genetic dissection of the Ca²⁺ -mediated adaptive mechanisms in PAMP-triggered immunity.

Lysolipid Chain Length Switches Agonistic to Antagonistic G Protein-Coupled Receptor Modulation

Activation of lysolipid-sensitive G protein-coupled receptors (GPCR) depends not only on lysolipid class but also on the length and degree of saturation of their respective hydrophobic tails. Positive regulation of these signaling networks caused by the lipid chain length specificity of upstream phospholipases is firmly established. Nonagonistic lysolipid homologues, featuring incompatible lipid tails, have been suggested to indirectly modulate GPCR signaling by delaying agonist catabolism. Nonetheless, recent results seem inconsistent with this hypothesis. Utilizing a simplified lysolipid-GPCR signaling assay based on the established lysophosphatidylglucoside-GPR55 signaling axis in primary sensory neurons, we demonstrate that short-chain ligand homologues directly modulate receptor activation via a potent competitive antagonistic activity. Considering the well-documented tissue-specific concentration of lysolipid homologues, we propose that endogenous lysolipids with insufficient chain length for stable receptor activation exert an antagonistic activity, effectively representing a negative control mechanism for GPCR-associated lysolipid signaling.

Cocktail Napkin Presentations: Design of an Activity to Enhance Undergraduate Communication and Critical Evaluation of Neuroscience Primary Literature

Distilling complex neuroscientific ideas in a succinct and elegant way is an art. The distilled product must have smoothly flowing logic, communicate a substantial body of knowledge, and be easily digestible by the audience. At the same time, the essence of scientific accuracy and experimental design cannot be lost in the distillation process. When undergraduates encounter primary literature for the first time, they are often stifled by its overpowering complexity and astringent technicality, but can quickly learn how exciting and interesting some of their subtle findings can be. Here, the design of a novel learning activity is presented that utilizes a cocktail napkin to synthesize the knowledge and skills required for fluidity in neuroscience primary literature. The activity was implemented within the context of an upper-level developmental neurobiology course for biology majors. The activity was designed specifically to increase neuroscience literacy and oral communication. The activity appeared to address a needed shift in students' attitudes to reading primary literature, and students additionally remarked how deeply engaged they were with the literature. When paired with mentored instruction, students' values toward neuroscience appeared to change as they learned to produce distillations that were rich in content and delightful to the scientific mind.

Inositol 1,4,5-Trisphosphate Receptor Type 3 Regulates Neuronal Growth Cone Sensitivity to Guidance Signals

During neurodevelopment, the growth cone deciphers directional information from extracellular guidance cues presented as shallow concentration gradients via signal amplification. However, it remains unclear how the growth cone controls this amplification process during its navigation through an environment in which basal cue concentrations vary widely. Here, we identified inositol 1,4,5-trisphosphate (IP₃) receptor type 3 as a regulator of axonal sensitivity to guidance cues in vitro and in vivo. Growth cones lacking the type 3 subunit are hypersensitive to nerve growth factor (NGF), an IP₃-dependent attractive cue, and incapable of turning toward normal concentration ranges of NGF to which wild-type growth cones respond. This is due to globally, but not asymmetrically, activated Ca²⁺ signaling in the hypersensitive growth cones. Remarkably, lower NGF concentrations can polarize growth cones for turning if IP₃ receptor type 3 is deficient. These data suggest a subtype-specific IP₃ receptor function in sensitivity adjustment during axon navigation.

•

IP₃ receptor type 3 (IP₃R3) controls axonal sensitivity to IP₃-based guidance cues

•

IP₃R3^−/− growth cones are not attracted to NGF due to global Ca²⁺ responses

•

Lower NGF concentrations can polarize IP₃R3^−/− growth cones for attractive turning

•

NGF knockdown in vivo can revert abnormal trajectory of IP₃R3^−/− axons

Receptor-specific interactome as a hub for rapid cue-induced selective translation in axons

Extrinsic cues trigger the local translation of specific mRNAs in growing axons via cell surface receptors. The coupling of ribosomes to receptors has been proposed as a mechanism linking signals to local translation but it is not known how broadly this mechanism operates, nor whether it can selectively regulate mRNA translation. We report that receptor-ribosome coupling is employed by multiple guidance cue receptors and this interaction is mRNA-dependent. We find that different receptors associate with distinct sets of mRNAs and RNA-binding proteins. Cue stimulation of growing Xenopus retinal ganglion cell axons induces rapid dissociation of ribosomes from receptors and the selective translation of receptor-specific mRNAs. Further, we show that receptor-ribosome dissociation and cue-induced selective translation are inhibited by co-exposure to translation-repressive cues, suggesting a novel mode of signal integration. Our findings reveal receptor-specific interactomes and suggest a generalizable model for cue-selective control of the local proteome.

A rationally designed NRP1-independent superagonist SEMA3A mutant is an effective anticancer agent

Vascular normalizing strategies, aimed at ameliorating blood vessel perfusion and lessening tissue hypoxia, are treatments that may improve the outcome of cancer patients. Secreted class 3 semaphorins (SEMA3), which are thought to directly bind neuropilin (NRP) co-receptors that, in turn, associate with and elicit plexin (PLXN) receptor signaling, are effective normalizing agents of the cancer vasculature. Yet, SEMA3A was also reported to trigger adverse side effects via NRP1. We rationally designed and generated a safe, parenterally deliverable, and NRP1-independent SEMA3A point mutant isoform that, unlike its wild-type counterpart, binds PLXNA4 with nanomolar affinity and has much greater biochemical and biological activities in cultured endothelial cells. In vivo, when parenterally administered in mouse models of pancreatic cancer, the NRP1-independent SEMA3A point mutant successfully normalized the vasculature, inhibited tumor growth, curbed metastatic dissemination, and effectively improved the supply and anticancer activity of chemotherapy. Mutant SEMA3A also inhibited retinal neovascularization in a mouse model of age-related macular degeneration. In summary, mutant SEMA3A is a vascular normalizing agent that can be exploited to treat cancer and, potentially, other diseases characterized by pathological angiogenesis.

Micropatterned nanolayers immobilized with nerve growth factor for neurite formation of PC12 cells

BACKGROUND

Nerve regeneration is important for the treatment of degenerative diseases and neurons injured by accidents. Nerve growth factor (NGF) has been previously conjugated to materials for promotion of neurogenesis.

MATERIALS AND METHODS

Photoreactive gelatin was prepared by chemical coupling of gelatin with azidobenzoic acid (P-gel), and then NGF was immobilized on substrates in the presence or absence of micropatterned photomasks. UV irradiation induced crosslinking reactions of P-gel with itself, NGF, and the plate for immobilization.

RESULTS

By adjustment of the P-gel concentration, the nanometer-order height of micropatterns was controlled. NGF was quantitatively immobilized with increasing amounts of P-gel. Immobilized NGF induced neurite outgrowth of PC12 cells, a cell line derived from a pheochromocytoma of the rat adrenal medulla, at the same level as soluble NGF. The immobilized NGF showed higher thermal stability than the soluble NGF and was repeatedly used without loss of biological activity. The 3D structure (height of the formed micropattern) regulated the behavior of neurite guidance. As a result, the orientation of neurites was regulated by the stripe pattern width.

CONCLUSION

The micropattern-immobilized NGF nanolayer biochemically and topologically regulated neurite formation.

A crucial role for Arf6 in the response of commissural axons to Slit

A switch in the response of commissural axons to the repellent Slit is crucial for ensuring that they cross the ventral midline only once. However, the underlying mechanisms remain to be elucidated. We have found that both endocytosis and recycling of Robo1 receptor are crucial for modulating Slit sensitivity in vertebrate commissural axons. Robo1 endocytosis and its recycling back to the cell surface maintained the stability of axonal Robo1 during Slit stimulation. We identified Arf6 guanosine triphosphatase and its activators, cytohesins, as previously unknown components in Slit-Robo1 signalling in vertebrate commissural neurons. Slit-Robo1 signalling activated Arf6. The Arf6-deficient mice exhibited marked defects in commissural axon midline crossing. Our data showed that a Robo1 endocytosis-triggered and Arf6-mediated positive-feedback strengthens the Slit response in commissural axons upon their midline crossing. Furthermore, the cytohesin-Arf6 pathways modulated this self-enhancement of the Slit response before and after midline crossing, resulting in a switch that reinforced robust regulation of axon midline crossing. Our study provides insights into endocytic trafficking-mediated mechanisms for spatiotemporally controlled axonal responses and uncovers new players in the midline switch in Slit responsiveness of commissural axons.

Cue-Polarized Transport of β-actin mRNA Depends on 3'UTR and Microtubules in Live Growth Cones

Guidance cues trigger fast responses in axonal growth cones such as directional turning and collapse that require local protein synthesis. An attractive cue-gradient, such as Netrin-1, triggers de novo synthesis of β-actin localized to the near-side compartment of the growth cone that promotes F-actin assembly and attractive steering. How this precise spatial asymmetry in mRNA translation arises across the small expanse of the growth cone is poorly understood. Pre-localized mRNAs in the vicinity of activated receptors could be selectively translated and/or new mRNAs could be trafficked into the area. Here we have performed live imaging of fluorescent-tagged β-actin mRNA to investigate mRNA trafficking dynamics in Xenopus retinal ganglion cell (RGC) axons and growth cones in response to Netrin-1. A Netrin-1 gradient was found to elicit the transport of β-actin mRNA granules to the near-side of growth cones within a 4-7 min window. This polarized mRNA trafficking depended on the 3' untranslated region (UTR) since mRNA-Δ3'UTR mutant failed to exhibit cue-induced localization. Global application of Netrin-1 significantly increased the anterograde movement of β-actin mRNA along axons and also promoted microtubule-dependent mRNA excursions from the central domain of the growth cone into the periphery (filopodia and lamellipodia). Dual channel imaging revealed β-actin mRNA riding behind the microtubule plus-end tracking protein, EB1, in movements along dynamic microtubules into filopodia. The mRNA-EB1 movements were unchanged by a Netrin-1 gradient indicating the dynamic microtubules themselves do not underlie the cue-induced polarity of RNA movement. Finally, fast-moving elongated "worm-like" trains of Cy3-RNA, distinct from mitochondria, were seen transporting RNA along axons in vitro and in vivo suggesting the existence of a novel transport organelle. Overall, the results provide evidence that the axonal trafficking of β-actin mRNA can be regulated by the guidance cue Netrin-1 to transduce the polarity of an extracellular stimulus and that the 3'UTR is essential for this cue-induced regulation.

Stepwise wiring of the Drosophila olfactory map requires specific Plexin B levels

The precise assembly of a neural circuit involves many consecutive steps. The conflict between a limited number of wiring molecules and the complexity of the neural network impels each molecule to execute multiple functions at different steps. Here, we examined the cell-type specific distribution of endogenous levels of axon guidance receptor Plexin B (PlexB) in the developing antennal lobe, the first olfactory processing center in Drosophila. We found that different classes of olfactory receptor neurons (ORNs) express PlexB at different levels in two wiring steps – axonal trajectory choice and subsequent target selection. In line with its temporally distinct patterns, the proper levels of PlexB control both steps in succession. Genetic interactions further revealed that the effect of high-level PlexB is antagonized by its canonical partner Sema2b. Thus, PlexB plays a multifaceted role in instructing the assembly of the Drosophila olfactory circuit through temporally-regulated expression patterns and expression level-dependent effects.

Semaphorin 3A as an inhibitive factor for migration of olfactory ensheathing cells through cofilin activation is involved in formation of olfactory nerve layer

Olfactory ensheathing cells (OECs) migrate from olfactory epithelium towards olfactory bulb (OB), contributing to formation of the presumptive olfactory nerve layer during development. However, it remains unclear that molecular mechanism of regulation of OEC migration in OB. In the present study, we found that OECs highly expressed the receptors of semaphorin 3A (Sema3A) in vitro and in vivo, whereas Sema3A displayed a gradient expression pattern with higher in inner layer of OB and lower in outer layer of OB. Furthermore, the collapse assays, Boyden chamber migration assays and single-cell migration assays showed that Sema3A induced the collapse of leading front of OECs and inhibited OEC migration. Thirdly, the leading front of OECs exhibited adaptation in a protein synthesis-independent manner, and endocytosis-dependent manner during Sema3A-induced OEC migration. Finally, Sema3A-induced collapse of leading front was required the decrease of focal adhesion and a retrograde F-actin flow in a cofilin activation-dependent manner. Taken together, these results demonstrate that Sema3A as an inhibitive migratory factor for OEC migration through cofilin activation is involved in the formation of olfactory nerve layer.

Identification of the extracellular matrix protein Fibulin-2 as a regulator of spinal nerve organization

During amniote peripheral nervous system development, segmentation ensures the correct patterning of the spinal nerves relative to the vertebral column. Along the antero-posterior (rostro-caudal) axis, each somite-derived posterior half-sclerotome expresses repellent molecules to restrict axon growth and neural crest migration to the permissive anterior half-segment. To identify novel regulators of spinal nerve patterning, we investigated the differential gene expression of anterior and posterior half-sclerotomes in the chick embryo by RNA-sequencing. Several genes encoding extracellular matrix proteins were found to be enriched in either anterior (e.g. Tenascin-C, Laminin alpha 4) or posterior (e.g. Fibulin-2, Fibromodulin, Collagen VI alpha 2) half-sclerotomes. Among them, the extracellular matrix protein Fibulin-2 was found specifically restricted to the posterior half-sclerotome. By using in ovo ectopic expression in chick somites, we found that Fibulin-2 modulates spinal axon growth trajectories in vivo. While no intrinsic axon repellent activity of Fibulin-2 was found, we showed that it enhances the growth cone repulsive activity of Semaphorin 3A in vitro. Some molecules regulating axon growth during development are found to be upregulated in the adult central nervous system (CNS) following traumatic injury. Here, we found increased Fibulin-2 protein levels in reactive astrocytes at the lesion site of a mouse model of CNS injury. Together, these results suggest that the developing vertebral column and the adult CNS share molecular features that control axon growth and plasticity, which may open up the possibility for the identification of novel therapeutic targets for brain and spinal cord injury.

Axonal mRNA transport and translation at a glance

Localization and translation of mRNAs within different subcellular domains provides an important mechanism to spatially and temporally introduce new proteins in polarized cells. Neurons make use of this localized protein synthesis during initial growth, regeneration and functional maintenance of their axons. Although the first evidence for protein synthesis in axons dates back to 1960s, improved methodologies, including the ability to isolate axons to purity, highly sensitive RNA detection methods and imaging approaches, have shed new light on the complexity of the transcriptome of the axon and how it is regulated. Moreover, these efforts are now uncovering new roles for locally synthesized proteins in neurological diseases and injury responses. In this Cell Science at a Glance article and the accompanying poster, we provide an overview of how axonal mRNA transport and translation are regulated, and discuss their emerging links to neurological disorders and neural repair.

Visualizing and Characterizing Semaphorin Endocytic Events Using Quantum Dot-Conjugated Proteins

Neurons can endocytose soluble semaphorins to either initiate or interrupt signaling at the cell membrane. Depending on the cell type and even on the specific subcellular domain, the endocytic process will differ in intensity, speed, and modality, and will subsequently facilitate diverse actions of semaphorin molecules. Therefore, in order to understand the physiology of guidance cues like semaphorins it is important to visualize endocytic events with good spatial and temporal resolution. Here, we describe methods to visualize endocytosed Semaphorin3A (Sema3A) molecules and to characterize the rate and pathway of internalization in primary rat neuronal cultures using semiconductor quantum dot nanoparticles (Q-dots).

Ephrin-A/EphA specific co-adaptation as a novel mechanism in topographic axon guidance

Genetic hardwiring during brain development provides computational architectures for innate neuronal processing. Thus, the paradigmatic chick retinotectal projection, due to its neighborhood preserving, topographic organization, establishes millions of parallel channels for incremental visual field analysis. Retinal axons receive targeting information from quantitative guidance cue gradients. Surprisingly, novel adaptation assays demonstrate that retinal growth cones robustly adapt towards ephrin-A/EphA forward and reverse signals, which provide the major mapping cues. Computational modeling suggests that topographic accuracy and adaptability, though seemingly incompatible, could be reconciled by a novel mechanism of coupled adaptation of signaling channels. Experimentally, we find such 'co-adaptation' in retinal growth cones specifically for ephrin-A/EphA signaling. Co-adaptation involves trafficking of unliganded sensors between the surface membrane and recycling endosomes, and is presumably triggered by changes in the lipid composition of membrane microdomains. We propose that co-adaptative desensitization eventually relies on guidance sensor translocation into cis-signaling endosomes to outbalance repulsive trans-signaling.

Expanding Axonal Transcriptome Brings New Functions for Axonally Synthesized Proteins in Health and Disease

Intra-axonal protein synthesis has been shown to play critical roles in both development and repair of axons. Axons provide long-range connectivity in the nervous system, and disruption of their function and/or structure is seen in several neurological diseases and disorders. Axonally synthesized proteins or losses in axonally synthesized proteins contribute to neurodegenerative diseases, neuropathic pain, viral transport, and survival of axons. Increasing sensitivity of RNA detection and quantitation coupled with methods to isolate axons to purity has shown that a surprisingly complex transcriptome exists in axons. This extends across different species, neuronal populations, and physiological conditions. These studies have helped define the repertoire of neuronal mRNAs that can localize into axons and imply previously unrecognized functions for local translation in neurons. Here, we review the current state of transcriptomics studies of isolated axons, contrast axonal mRNA profiles between different neuronal types and growth states, and discuss how mRNA transport into and translation within axons contribute to neurological disorders.

Single Molecule Translation Imaging Visualizes the Dynamics of Local β-Actin Synthesis in Retinal Axons

Local mRNA translation occurs in growing axons enabling precise control of the proteome in response to signals. To measure quantitatively the spatiotemporal dynamics of protein synthesis in growth cones, we further developed a technique for single molecule translation imaging (SMTI). We report that Netrin-1 triggers a burst of β-actin synthesis at multiple non-repetitive sites, particularly in the periphery. The response is remarkably rapid starting within 20 seconds of cue application.

Post-endocytic sorting of Plexin-D1 controls signal transduction and development of axonal and vascular circuits

Local endocytic events involving receptors for axon guidance cues play a central role in controlling growth cone behaviour. Yet, little is known about the fate of internalized receptors, and whether the sorting events directing them to distinct endosomal pathways control guidance decisions. Here, we show that the receptor Plexin-D1 contains a sorting motif that interacts with the adaptor protein GIPC1 to facilitate transport to recycling endosomes. This sorting process promotes colocalization of Plexin-D1 with vesicular pools of active R-ras, leading to its inactivation. In the absence of interaction with GIPC1, missorting of Plexin-D1 results in loss of signalling activity. Consequently, Gipc1 mutant mice show specific defects in axonal projections, as well as vascular structures, that rely on Plexin-D1 signalling for their development. Thus, intracellular sorting steps that occur after receptor internalization by endocytosis provide a critical level of control of cellular responses to guidance signals.

Molecular mechanisms controlling axonal growth cone behaviour are only partially understood. Here the authors reveal a role of an adaptor protein GIPC1 in Plexin-D1 receptor recycling, and show that this process is required for axon track formation and vascular patterning in mice.

Neuronal Cell Bodies Remotely Regulate Axonal Growth Response to Localized Netrin-1 Treatment via Second Messenger and DCC Dynamics

Netrin-1 modulates axonal growth direction and speed. Its best characterized receptor, Deleted in Colorectal Cancer (DCC), is localized to growth cones, but also observed in the cell bodies. We hypothesized that cell bodies sense Netrin-1 and contribute to axon growth rate modulation, mediated by the second messenger system. We cultured mouse cortical neurons in microfluidic devices to isolate distal axon and cell body microenvironments. Compared to isolated axonal treatment, global Netrin-1 treatment decreased the axon elongation rate and affected the dynamics of total and membranous DCC, calcium, and cyclic nucleotides. Signals induced by locally applied Netrin-1 propagated in both anterograde and retrograde directions, demonstrated by the long-range increase in DCC and by the increased frequency of calcium transients in cell bodies, evoked by axonal Netrin-1. Blocking the calcium efflux from endoplasmic reticulum suppressed the membranous DCC response. Our findings support the notion that neurons sense Netrin-1 along their entire lengths in making axonal growth decisions.

Visualization of Clathrin-Mediated Endocytosis During Semaphorin-Guided Axonal Growth

Semaphorin3A (Sema3A) guides axonal growth during neuronal network development. Accumulating evidence indicates that Sema3A-induced growth cone collapse and repulsion involve endocytic membrane trafficking in the growth cone. It is now possible to visualize endocytic processes in living cells using total internal reflection fluorescence microscopy (TIRFM), a powerful tool for imaging dynamic subcellular events at the plasma membrane. In this chapter, we describe a method for TIRFM observation and analysis of clathrin-mediated endocytosis in growth cones of chicken dorsal root ganglion neurons that receive an extracellular concentration gradient of Sema3A in a culture medium.

Multi-phasic bi-directional chemotactic responses of the growth cone

The nerve growth cone is bi-directionally attracted and repelled by the same cue molecules depending on the situations, while other non-neural chemotactic cells usually show uni-directional attraction or repulsion toward their specific cue molecules. However, how the growth cone differs from other non-neural cells remains unclear. Toward this question, we developed a theory for describing chemotactic response based on a mathematical model of intracellular signaling of activator and inhibitor. Our theory was first able to clarify the conditions of attraction and repulsion, which are determined by balance between activator and inhibitor, and the conditions of uni- and bi-directional responses, which are determined by dose-response profiles of activator and inhibitor to the guidance cue. With biologically realistic sigmoidal dose-responses, our model predicted tri-phasic turning response depending on intracellular Ca²⁺ level, which was then experimentally confirmed by growth cone turning assays and Ca²⁺ imaging. Furthermore, we took a reverse-engineering analysis to identify balanced regulation between CaMKII (activator) and PP1 (inhibitor) and then the model performance was validated by reproducing turning assays with inhibitions of CaMKII and PP1. Thus, our study implies that the balance between activator and inhibitor underlies the multi-phasic bi-directional turning response of the growth cone.

Tracking Differential Endocytosis and Trafficking of Semaphorin Receptor Complexes in Responding Nerve Growth Cones

The study of receptor endocytosis is important to our understanding of the signal transduction events initiated by axon guidance cues in growth cones. Fab fragments of antibodies to guidance receptors and endocytic cargoes like transferrin and cholera toxin-B are the tools of choice for studying the dynamics of receptor internalization and intracellular trafficking by different pathways. We describe a method where in vitro cultures of growth cones are incubated with these ligands in the presence or absence of Sema3A, followed by stripping of remaining ligand on cell-surface and analysis by immunofluorescence techniques. These techniques can be employed for studying the endocytosis of any axon guidance receptor in response to attractive or repulsive guidance cues and, in particular, to allow the differential trafficking of specific receptor components to be revealed.

ESCRT-II controls retinal axon growth by regulating DCC receptor levels and local protein synthesis

Endocytosis and local protein synthesis (LPS) act coordinately to mediate the chemotropic responses of axons, but the link between these two processes is poorly understood. The endosomal sorting complex required for transport (ESCRT) is a key regulator of cargo sorting in the endocytic pathway, and here we have investigated the role of ESCRT-II, a critical ESCRT component, in Xenopus retinal ganglion cell (RGC) axons. We show that ESCRT-II is present in RGC axonal growth cones (GCs) where it co-localizes with endocytic vesicle GTPases and, unexpectedly, with the Netrin-1 receptor, deleted in colorectal cancer (DCC). ESCRT-II knockdown (KD) decreases endocytosis and, strikingly, reduces DCC in GCs and leads to axon growth and guidance defects. ESCRT-II-depleted axons fail to turn in response to a Netrin-1 gradient in vitro and many axons fail to exit the eye in vivo. These defects, similar to Netrin-1/DCC loss-of-function phenotypes, can be rescued in whole (in vitro) or in part (in vivo) by expressing DCC. In addition, ESCRT-II KD impairs LPS in GCs and live imaging reveals that ESCRT-II transports mRNAs in axons. Collectively, our results show that the ESCRT-II-mediated endocytic pathway regulates both DCC and LPS in the axonal compartment and suggest that ESCRT-II aids gradient sensing in GCs by coupling endocytosis to LPS.

Membrane Trafficking in Neuronal Development: Ins and Outs of Neural Connectivity

During development, neurons progress through rapid yet stereotypical shape changes to achieve proper neuronal connectivity. This morphological progression requires carefully orchestrated plasma membrane expansion, insertion of membrane components including receptors for extracellular cues into the plasma membrane and removal and trafficking of membrane materials and proteins to specific locations. This review outlines the cellular machinery of membrane trafficking that play an integral role in neuronal cell shape change and function from initial neurite formation to pathway navigation and synaptogenesis.

Semaphorin-3a, neuropilin-1 and plexin-A1 in prosthetic-particle induced bone loss

Peri-prosthetic osteolysis (PPO) occurs in response to prosthetic wear particles causing an inflammatory reaction in the surrounding tissue that leads to subsequent bone loss. Semaphorin-3a (SEM3A), neuropilin-1 (NRP1) and plexin-A1 (PLEXA1) are axonal guidance molecules that have been recently implicated in regulating bone metabolism. This study investigated SEM3A, NRP1 and PLEXA1 protein and mRNA expression in human PPO tissue and polyethylene (PE) particle-stimulated human peripheral blood mononuclear cell (PBMC)-derived osteoclasts in vitro. In addition, the effects of tumour necrosis factor alpha (TNFα) on cultured osteoclasts was assessed. In PPO tissues, a granular staining pattern of SEM3A and NRP1 was observed within large multi-nucleated cells that contained prosthetic wear particles. Immunofluorescent staining confirmed the expression of SEM3A, NRP1 and PLEXA1 in large multi-nucleated human osteoclasts in vitro. Furthermore, SEM3A, NRP1 and PLEXA1 mRNA levels progressively increased throughout osteoclast differentiation induced by receptor activator of nuclear factor κB ligand (RANKL), and the presence of PE particles further increased mRNA expression of all three molecules. Soluble SEM3A was detected in human osteoclast culture supernatant at days 7 and 17 of culture, as assessed by ELISA. TNFα treatment for 72h markedly decreased the mRNA expression of SEM3A, NRP1 and PLEXA1 by human osteoclasts in vitro. Our findings suggest that SEM3A, NRP1 and PLEXA1 may have important roles in PPO, and their interactions, alone or as a complex, may have a role in pathological bone loss progression.

STATEMENT OF SIGNIFICANCE

Peri-prosthetic osteolysis occurs in response to prosthetic wear particles causing an inflammatory reaction in the surrounding tissue that leads to subsequent bone loss. The rate of hip and knee arthroplasty is increasing by at least 5% per year. However, these joint replacements have a finite lifespan, with data from the National Joint Replacement Registry (Australia) showing that the major cause of failure of total hip replacements is aseptic loosening. In aseptic loosening, wear particles liberated from prostheses are phagocytosed by macrophages, leading to release of inflammatory cytokines and up-regulation of osteoclast formation and activity. Semaphorin-3a, neuropilin-1 and plexin-A1 are axonal guidance molecules that have been recently implicated in regulating bone metabolism. This is the first report to show that these molecules may be involved in the implant failure.

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.

Slit-Dependent Endocytic Trafficking of the Robo Receptor Is Required for Son of Sevenless Recruitment and Midline Axon Repulsion

Understanding how axon guidance receptors are activated by their extracellular ligands to regulate growth cone motility is critical to learning how proper wiring is established during development. Roundabout (Robo) is one such guidance receptor that mediates repulsion from its ligand Slit in both invertebrates and vertebrates. Here we show that endocytic trafficking of the Robo receptor in response to Slit-binding is necessary for its repulsive signaling output. Dose-dependent genetic interactions and in vitro Robo activation assays support a role for Clathrin-dependent endocytosis, and entry into both the early and late endosomes as positive regulators of Slit-Robo signaling. We identify two conserved motifs in Robo's cytoplasmic domain that are required for its Clathrin-dependent endocytosis and activation in vitro; gain of function and genetic rescue experiments provide strong evidence that these trafficking events are required for Robo repulsive guidance activity in vivo. Our data support a model in which Robo's ligand-dependent internalization from the cell surface to the late endosome is essential for receptor activation and proper repulsive guidance at the midline by allowing recruitment of the downstream effector Son of Sevenless in a spatially constrained endocytic trafficking compartment.

Netrin-1 directs dendritic growth and connectivity of vertebrate central neurons in vivo

BACKGROUND

Netrins are a family of extracellular proteins that function as chemotropic guidance cues for migrating cells and axons during neural development. In the visual system, netrin-1 has been shown to play a key role in retinal ganglion cell (RGC) axon growth and branching at the target, where presynaptic RGC axons form partnerships with the dendrites of tectal neurons. However, the signals that guide the connections between RGC axons and their postsynaptic partners are yet unknown. Here, we explored dynamic cellular mechanisms by which netrin-1 influences visual circuit formation, particularly those that impact postsynaptic neuronal morphology and connectivity during retinotectal wiring.

RESULTS

Time-lapse in vivo imaging of individual Xenopus laevis optic tectal neurons co-expressing tdTomato and PSD95-GFP revealed rapid remodeling and reorganization of dendritic arbors following acute manipulations in netrin-1 levels. Effects of altered netrin signaling on developing dendritic arbors of tectal neurons were distinct from its effects on presynaptic RGC axons. Within 4 h of treatment, tectal injection of recombinant netrin-1 or sequestration of endogenous netrin with an UNC-5 receptor ectodomain induced significant changes in the directionality and orientation of dendrite growth and in the maintenance of already established dendrites, demonstrating that relative levels of netrin are important for these functions. In contrast, altering DCC-mediated netrin signaling with function-blocking antibodies induced postsynaptic specialization remodeling and changed growth directionality of already established dendrites. Reducing netrin signaling also decreased avoidance behavior in a visually guided task, suggesting that netrin is essential for emergent visual system function.

CONCLUSIONS

These in vivo findings together with the patterns of expression of netrin and its receptors reveal an important role for netrin in the early growth and guidance of vertebrate central neuron dendritic arbors. Collectively, our studies indicate that netrin shapes both pre- and postsynaptic arbor morphology directly and in multiple ways at stages critical for functional visual system development.

The cytoskeleton-associated protein SCHIP1 is involved in axon guidance, and is required for piriform cortex and anterior commissure development

SCHIP1 is a cytoplasmic partner of cortical cytoskeleton ankyrins. The IQCJ-SCHIP1 isoform is a component of axon initial segments and nodes of Ranvier of mature axons in peripheral and central nervous systems, where it associates with membrane complexes comprising cell adhesion molecules. SCHIP1 is also expressed in the mouse developing central nervous system during embryonic stages of active axonogenesis. Here, we identify a new and early role for SCHIP1 during axon development and establishment of the anterior commissure (AC). The AC is composed of axons from the piriform cortex, the anterior olfactory nucleus and the amygdala. Schip1 mutant mice displayed early defects in AC development that might result from impaired axon growth and guidance. In addition, mutant mice presented a reduced thickness of the piriform cortex, which affected projection neurons in layers 2/3 and was likely to result from cell death rather than from impairment of neuron generation or migration. Piriform cortex neurons from E14.5 mutant embryos displayed axon initiation/outgrowth delay and guidance defects in vitro. The sensitivity of growth cones to semaphorin 3F and Eph receptor B2, two repulsive guidance cues crucial for AC development, was increased, providing a possible basis for certain fiber tract alterations. Thus, our results reveal new evidence for the involvement of cortical cytoskeleton-associated proteins in the regulation of axon development and their importance for the formation of neuronal circuits.

Differential requirement of F-actin and microtubule cytoskeleton in cue-induced local protein synthesis in axonal growth cones

BACKGROUND

Local protein synthesis (LPS) via receptor-mediated signaling plays a role in the directional responses of axons to extrinsic cues. An intact cytoskeleton is critical to enact these responses, but it is not known whether the two major cytoskeletal elements, F-actin and microtubules, have any roles in regulating axonal protein synthesis.

RESULTS

Here, we show that pharmacological disruption of either microtubules or actin filaments in growth cones blocks netrin-1-induced de novo synthesis of proteins, as measured by metabolic incorporation of labeled amino acids, implicating both elements in axonal synthesis. However, comparative analysis of the activated translation initiation regulator, eIF4E-BP1, revealed a striking difference in the point of action of the two elements: actin disruption completely inhibited netrin-1-induced eIF4E-BP1 phosphorylation while microtubule disruption had no effect. An intact F-actin, but not microtubule, cytoskeleton was also required for netrin-1-induced activation of the PI3K/Akt/mTOR pathway, upstream of translation initiation. Downstream of translation initiation, microtubules were required for netrin-1-induced activation of eukaryotic elongation factor 2 kinase (eEF2K) and eEF2.

CONCLUSIONS

Taken together, our results show that while actin and microtubules are both crucial for cue-induced axonal protein synthesis, they serve distinct roles with F-actin being required for the initiation of translation and microtubules acting later at the elongation step.

Netrins and Frazzled/DCC promote the migration and mesenchymal to epithelial transition of Drosophila midgut cells

Mesenchymal-epithelial transitions (METs) are important in both development and the growth of secondary tumours. Although the molecular basis for epithelial polarity is well studied, less is known about the cues that induce MET. Here we show that Netrins, well known as chemotropic guidance factors, provide a basal polarising cue during the Drosophila midgut MET. Both netrinA and netrinB are expressed in the visceral mesoderm, the substrate upon which midgut cells migrate, while their receptor frazzled (fra) is expressed in midgut cells. Netrins are required to polarise Fra to the basal surface, and Netrins and Fra undergo mutually-dependent endocytosis, with Fra subsequently trafficking to late endosomes. Mutations to fra and netrins affect both migration and MET but to different degrees. Loss of fra strongly delays migration, midgut cells fail to extend protrusions, and apico-basal polarisation of proteins and epithelium formation is inhibited. In netrin mutants, the migration phenotype is weaker and cells still extend protrusions. However, apico-basal polarisation of proteins, including Fra, and FActin is greatly disrupted and a monolayer fails to form. Delocalised accumulations of FActin are prevalent in netrin mutants but not fra mutants suggesting delocalised Fra may disrupt the MET. βPS localisation is also affected in netrin mutants in that a basal gradient is reduced while localisation to the midgut/VM interface is increased. Since a similar effect is seen when endocytosis is inhibited, Netrin and Fra may regulate Integrin turnover. The results suggest Netrin-dependent basal polarisation of Fra is critical for the formation of an epithelium.

Semaphorin signalling during development

Semaphorins are secreted and membrane-associated proteins that regulate many different developmental processes, including neural circuit assembly, bone formation and angiogenesis. Trans and cis interactions between semaphorins and their multimeric receptors trigger intracellular signal transduction networks that regulate cytoskeletal dynamics and influence cell shape, differentiation, motility and survival. Here and in the accompanying poster we provide an overview of the molecular biology of semaphorin signalling within the context of specific cell and developmental processes, highlighting the mechanisms that act to fine-tune, diversify and spatiotemporally control the effects of semaphorins.

Remote control of gene function by local translation

The subcellular position of a protein is a key determinant of its function. Mounting evidence indicates that RNA localization, where specific mRNAs are transported subcellularly and subsequently translated in response to localized signals, is an evolutionarily conserved mechanism to control protein localization. On-site synthesis confers novel signaling properties to a protein and helps to maintain local proteome homeostasis. Local translation plays particularly important roles in distal neuronal compartments, and dysregulated RNA localization and translation cause defects in neuronal wiring and survival. Here, we discuss key findings in this area and possible implications of this adaptable and swift mechanism for spatial control of gene function.

Rab5 and Rab4 regulate axon elongation in the Xenopus visual system

The elongation rate of axons is tightly regulated during development. Recycling of the plasma membrane is known to regulate axon extension; however, the specific molecules involved in recycling within the growth cone have not been fully characterized. Here, we investigated whether the small GTPases Rab4 and Rab5 involved in short-loop recycling regulate the extension of Xenopus retinal axons. We report that, in growth cones, Rab5 and Rab4 proteins localize to endosomes, which accumulate markers that are constitutively recycled. Fluorescence recovery after photo-bleaching experiments showed that Rab5 and Rab4 are recruited to endosomes in the growth cone, suggesting that they control recycling locally. Dynamic image analysis revealed that Rab4-positive carriers can bud off from Rab5 endosomes and move to the periphery of the growth cone, suggesting that both Rab5 and Rab4 contribute to recycling within the growth cone. Inhibition of Rab4 function with dominant-negative Rab4 or Rab4 morpholino and constitutive activation of Rab5 decreases the elongation of retinal axons in vitro and in vivo, but, unexpectedly, does not disrupt axon pathfinding. Thus, Rab5- and Rab4-mediated control of endosome trafficking appears to be crucial for axon growth. Collectively, our results suggest that recycling from Rab5-positive endosomes via Rab4 occurs within the growth cone and thereby supports axon elongation.

Regulation of mRNA transport, localization and translation in the nervous system of mammals (Review)

Post-transcriptional control of mRNA trafficking and metabolism plays a critical role in the actualization and fine tuning of the genetic program of cells, both in development and in differentiated tissues. Cis-acting signals, responsible for post-transcriptional regulation, reside in the RNA message itself, usually in untranslated regions, 5′ or 3′ to the coding sequence, and are recognized by trans-acting factors: RNA-binding proteins (RBPs) and/or non-coding RNAs (ncRNAs). ncRNAs bind short mRNA sequences usually present in the 3′-untranslated (3′-UTR) region of their target messages. RBPs recognize specific nucleotide sequences and/or secondary/tertiary structures. Most RBPs assemble on mRNA at the moment of transcription and shepherd it to its destination, somehow determining its final fate. Regulation of mRNA localization and metabolism has a particularly important role in the nervous system where local translation of pre-localized mRNAs has been implicated in developing axon and dendrite pathfinding, and in synapse formation. Moreover, activity-dependent mRNA trafficking and local translation may underlie long-lasting changes in synaptic efficacy, responsible for learning and memory. This review focuses on the role of RBPs in neuronal development and plasticity, as well as possible connections between ncRNAs and RBPs.

The central dogma decentralized: new perspectives on RNA function and local translation in neurons

The elaborate morphology of neurons together with the information processing that occurs in remote dendritic and axonal compartments makes the use of decentralized cell biological machines necessary. Recent years have witnessed a revolution in our understanding of signaling in neuronal compartments and the manifold functions of a variety of RNA molecules that regulate protein translation and other cellular functions. Here we discuss the view that mRNA localization and RNA-regulated and localized translation underlie many fundamental neuronal processes and highlight key issues for future experiments.

DRG axon elongation and growth cone collapse rate induced by Sema3A are differently dependent on NGF concentration

Regeneration of embryonic and adult dorsal root ganglion (DRG) sensory axons is highly impeded when they encounter neuronal growth cone-collapsing factor semaphorin3A (Sema3A). On the other hand, increasing evidence shows that DRG axon's regeneration can be stimulated by nerve growth factor (NGF). In this study, we aimed to evaluate whether increased NGF concentrations can counterweight Sema3A-induced inhibitory responses in 15-day-old mouse embryo (E15) DRG axons. The DRG explants were grown in Neurobasal-based medium with different NGF concentrations ranging from 0 to 100 ng/mL and then treated with Sema3A at constant 10 ng/mL concentration. To evaluate interplay between NGF and Sema3A number of DRG axons, axon outgrowth distance and collapse rate were measured. We found that the increased NGF concentrations abolish Sema3A-induced inhibitory effect on axon outgrowth, while they have no effect on Sema3A-induced collapse rate.

Integrins and cAMP mediate netrin-induced growth cone collapse

Growth cones integrate a remarkably complex concert of chemical cues to guide axons to their appropriate destinations. Recent work suggests that integrins contribute to axon guidance by interacting with a wide range of extracellular molecules including axon guidance molecules, by mechanisms that are not fully understood. Here, we describe an interaction between integrins and netrin-1 in growth cones that contributes to growth cone collapse. Our data show that netrin-1 causes growth cone collapse in a substratum-specific manner and is integrin-dependent. Netrin-1 causes collapse of cultured chick dorsal root ganglion (DRG) growth cones extending on high levels of laminin-1 (LN) but not growth cones extending on low levels of LN or on fibronectin. Blocking integrin function significantly decreases netrin-induced growth cone collapse on high LN. Netrin-1 and integrins interact on growth cones; netrin-1 causes integrin activation, a conformational shift to a high ligand-affinity state. Netrin-1 directly binds to integrin α3 and α6 peptides, further suggesting a netrin-integrin interaction. Interestingly, our data reveal that netrin-1 increases growth cone levels of cAMP in a substratum-specific manner and that netrin-induced growth cone collapse requires increased cAMP in combination with integrin activation. Manipulations that either decrease cAMP levels or integrin activation block netrin-induced collapse. These results imply a common mechanism for growth cone collapse and novel interactions between integrins, netrin-1 and cAMP that contribute to growth cone guidance.

Commissureless regulation of axon outgrowth across the midline is independent of Rab function

Nervous system function requires that neurons within neural circuits are connected together precisely. These connections form during the process of axon guidance whereby each neuron extends an axon that migrates, often large distances, through a complex environment to reach its synaptic target. This task can be simplified by utilising intermediate targets to divide the route into smaller sections. This requires that axons adapt their behaviour as they migrate towards and away from intermediate targets. In the central nervous system the midline acts as an intermediate target for commissural axons. In Drosophila commissural axons switch from attraction towards to extension away from the midline by regulating the levels of the Roundabout receptor on their cell surface. This is achieved by Commissureless which directs Roundabout to an intracellular compartment in the soma prior to reaching the midline. Once across the midline Roundabout is allowed to reach the surface and acts as a receptor for the repellent ligand Slit that is secreted by cells at the midline. Here we investigated candidate intracellular mechanisms that may facilitate the intracellular targeting of Commissureless and Roundabout within the soma of commissural neurons. Using modified forms of Commissureless or Rabs we show that neither ubiquitination nor Rab activity are necessary for the intracellular targeting of Commissureless. In addition we reveal that axon outgrowth of many populations of neurons within the Drosophila central nervous system is also independent of Rab activity.

Coupling of NF-protocadherin signaling to axon guidance by cue-induced translation

Cell adhesion molecules and diffusible cues both regulate axon pathfinding, yet how these two modes of signaling interact is poorly understood. The homophilic cell adhesion molecule NF-protocadherin (NFPC) is expressed in the mid-dorsal optic tract neuroepithelium and in the axons of developing retinal ganglion cells (RGC) in Xenopus laevis. Here we report that targeted disruption of NFPC function in RGC axons or the optic tract neuroepithelium results in unexpectedly localized pathfinding defects at the caudal turn in the mid-optic tract. Semaphorin 3A (Sema3A), which lies adjacent to this turn, stimulates rapid, protein synthesis-dependent increases in growth cone NFPC and its cofactor, TAF1, in vitro. In vivo, growth cones exhibit marked increases in NFPC translation reporter activity in this mid-optic tract region that are attenuated by blocking neuropilin-1 function. Our results suggest that translation-linked coupling between regionally localized diffusible cues and cell adhesion can help axons navigate discrete segments of the pathway.