Retinal Axon Response to Ephrin-As Shows a Graded, Concentration-Dependent Transition from Growth Promotion to Inhibition

Activation of multiple Eph receptors on neuronal membranes correlates with the onset of optic neuropathy

BACKGROUND

Optic neuropathy is a major cause of irreversible blindness, yet the molecular determinants that contribute to neuronal demise have not been fully elucidated. Several studies have identified 'ephrin signaling' as one of the most dysregulated pathways in the early pathophysiology of optic neuropathy with varied etiologies. Developmentally, gradients in ephrin signaling coordinate retinotopic mapping via repulsive modulation of cytoskeletal dynamics in neuronal membranes. Little is known about the role ephrin signaling plays in the post-natal visual system and its correlation with the onset of optic neuropathy.

METHODS

Postnatal mouse retinas were collected for mass spectrometry analysis for erythropoietin-producing human hepatocellular (Eph) receptors. Optic nerve crush (ONC) model was employed to induce optic neuropathy, and proteomic changes during the acute phase of neuropathic onset were analyzed. Confocal and super-resolution microscopy determined the cellular localization of activated Eph receptors after ONC injury. Eph receptor inhibitors assessed the neuroprotective effect of ephrin signaling modulation.

RESULTS

Mass spectrometry revealed expression of seven Eph receptors (EphA2, A4, A5, B1, B2, B3, and B6) in postnatal mouse retinal tissue. Immunoblotting analysis indicated a significant increase in phosphorylation of these Eph receptors 48 h after ONC. Confocal microscopy demonstrated the presence of both subclasses of Eph receptors within the retina. Stochastic optical reconstruction microscopy (STORM) super-resolution imaging combined with optimal transport colocalization analysis revealed a significant co-localization of activated Eph receptors with injured neuronal cells, compared to uninjured neuronal and/or injured glial cells, 48 h post-ONC. Eph receptor inhibitors displayed notable neuroprotective effects for retinal ganglion cells (RGCs) after six days of ONC injury.

CONCLUSIONS

Our findings demonstrate the functional presence of diverse Eph receptors in the postnatal mammalian retina, capable of modulating multiple biological processes. Pan-Eph receptor activation contributes to the onset of neuropathy in optic neuropathies, with preferential activation of Eph receptors on neuronal processes in the inner retina following optic nerve injury. Notably, Eph receptor activation precedes neuronal loss. We observed a neuroprotective effect on RGCs upon inhibiting Eph receptors. Our study highlights the importance of investigating this repulsive pathway in early optic neuropathies and provides a comprehensive characterization of the receptors present in the developed retina of mice, relevant to both homeostasis and disease processes.

Activation of Multiple Eph Receptors on Neuronal Membranes Correlates with The Onset of Traumatic Optic Neuropathy

Background

Optic neuropathy (ON) is a major cause of irreversible blindness, yet the molecular determinants that contribute to neuronal demise have not been fully elucidated. Several studies have identified 'ephrin signaling' as one of the most dysregulated pathways in the early pathophysiology of ON with varied etiologies. Developmentally, gradients in ephrin signaling coordinate retinotopic mapping via repulsive modulation of cytoskeletal dynamics in neuronal membranes. Little is known about the role ephrin signaling played in the post-natal visual system and its correlation with the onset of optic neuropathy.

Methods

Postnatal mouse retinas were collected for mass spectrometry analysis for Eph receptors. Optic nerve crush (ONC) model was employed to induce optic neuropathy, and proteomic changes during the acute phase of neuropathic onset were analyzed. Confocal and super-resolution microscopy determined the cellular localization of activated Eph receptors after ONC injury. Eph receptor inhibitors assessed the neuroprotective effect of ephrin signaling modulation.

Results

Mass spectrometry revealed expression of seven Eph receptors (EphA2, A4, A5, B1, B2, B3, and B6) in postnatal mouse retinal tissue. Immunoblotting analysis indicated a significant increase in phosphorylation of these Eph receptors 48 hours after ONC. Confocal microscopy demonstrated the presence of both subclasses of Eph receptors in the inner retinal layers. STORM super-resolution imaging combined with optimal transport colocalization analysis revealed a significant co-localization of activated Eph receptors with injured neuronal processes, compared to uninjured neuronal and/or injured glial cells, 48 hours post-ONC. Eph receptor inhibitors displayed notable neuroprotective effects after 6 days of ONC injury.

Conclusions

Our findings demonstrate the functional presence of diverse Eph receptors in the postnatal mammalian retina, capable of modulating multiple biological processes. Pan-Eph receptor activation contributes to the onset of neuropathy in ONs, with preferential activation of Eph receptors on neuronal processes in the inner retina following optic nerve injury. Notably, Eph receptor activation precedes neuronal loss. We observed neuroprotective effects upon inhibiting Eph receptors. Our study highlights the importance of investigating this repulsive pathway in early optic neuropathies and provides a comprehensive characterization of the receptors present in the developed retina of mice, relevant to both homeostasis and disease processes.

A Guide for the Multiplexed: The Development of Visual Feature Maps in the Brain

Neural maps are found ubiquitously in the brain, where they encode a wide range of behaviourally relevant features into neural space. Developmental studies have shown that animals devote a great deal of resources to establish consistently patterned organization in neural circuits throughout the nervous system, but what purposes maps serve beneath their often intricate appearance and composition is a topic of active debate and exploration. In this article, we review the general mechanisms of map formation, with a focus on the visual system, and then survey notable organizational properties of neural maps: the multiplexing of feature representations through a nested architecture, the interspersing of fine-scale heterogeneity within a globally smooth organization, and the complex integration at the microcircuit level that enables a high dimensionality of information encoding. Finally, we discuss the roles of maps in cortical functions, including input segregation, feature extraction and routing of circuit outputs for higher order processing, as well as the evolutionary basis for the properties we observe in neural maps.

Multiple Functions of Draxin/Netrin-1 Signaling in the Development of Neural Circuits in the Spinal Cord and the Brain

Axon guidance proteins play key roles in the formation of neural circuits during development. We previously identified an axon guidance cue, named draxin, that has no homology with other axon guidance proteins. Draxin is essential for the development of various neural circuits including the spinal cord commissure, corpus callosum, and thalamocortical projections. Draxin has been shown to not only control axon guidance through netrin-1 receptors, deleted in colorectal cancer (Dcc), and neogenin (Neo1) but also modulate netrin-1-mediated axon guidance and fasciculation. In this review, we summarize the multifaceted functions of draxin and netrin-1 signaling in neural circuit formation in the central nervous system. Furthermore, because recent studies suggest that the distributions and functions of axon guidance cues are highly regulated by glycoproteins such as Dystroglycan and Heparan sulfate proteoglycans, we discuss a possible function of glycoproteins in draxin/netrin-1-mediated axon guidance.

Eph-Ephrin Signaling Mediates Cross-Talk Within the Bone Microenvironment

Skeletal integrity is maintained through the tightly regulated bone remodeling process that occurs continuously throughout postnatal life to replace old bone and to repair skeletal damage. This is maintained primarily through complex interactions between bone resorbing osteoclasts and bone forming osteoblasts. Other elements within the bone microenvironment, including stromal, osteogenic, hematopoietic, endothelial and neural cells, also contribute to maintaining skeletal integrity. Disruption of the dynamic interactions between these diverse cellular systems can lead to poor bone health and an increased susceptibility to skeletal diseases including osteopenia, osteoporosis, osteoarthritis, osteomalacia, and major fractures. Recent reports have implicated a direct role for the Eph tyrosine kinase receptors and their ephrin ligands during bone development, homeostasis and skeletal repair. These membrane-bound molecules mediate contact-dependent signaling through both the Eph receptors, termed forward signaling, and through the ephrin ligands, referred to as reverse signaling. This review will focus on Eph/ ephrin cross-talk as mediators of hematopoietic and stromal cell communication, and how these interactions contribute to blood/ bone marrow function and skeletal integrity during normal steady state or pathological conditions.

EphA7 isoforms differentially regulate cortical dendrite development

The shape of a neuron facilitates its functionality within neural circuits. Dendrites integrate incoming signals from axons, receiving excitatory input onto small protrusions called dendritic spines. Therefore, understanding dendritic growth and development is fundamental for discerning neural function. We previously demonstrated that EphA7 receptor signaling during cortical development impacts dendrites in two ways: EphA7 restricts dendritic growth early and promotes dendritic spine formation later. Here, the molecular basis for this shift in EphA7 function is defined. Expression analyses reveal that EphA7 full-length (EphA7-FL) and truncated (EphA7-T1; lacking kinase domain) isoforms are dynamically expressed in the developing cortex. Peak expression of EphA7-FL overlaps with dendritic elaboration around birth, while highest expression of EphA7-T1 coincides with dendritic spine formation in early postnatal life. Overexpression studies in cultured neurons demonstrate that EphA7-FL inhibits both dendritic growth and spine formation, while EphA7-T1 increases spine density. Furthermore, signaling downstream of EphA7 shifts during development, such that in vivo inhibition of mTOR by rapamycin in EphA7-mutant neurons ameliorates dendritic branching, but not dendritic spine phenotypes. Finally, direct interaction between EphA7-FL and EphA7-T1 is demonstrated in cultured cells, which results in reduction of EphA7-FL phosphorylation. In cortex, both isoforms are colocalized to synaptic fractions and both transcripts are expressed together within individual neurons, supporting a model where EphA7-T1 modulates EphA7-FL repulsive signaling during development. Thus, the divergent functions of EphA7 during cortical dendrite development are explained by the presence of two variants of the receptor.

Wiring subcortical image-forming centers: Topography, laminar targeting, and map alignment

Efficient sensory processing is a complex and important function for species survival. As such, sensory circuits are highly organized to facilitate rapid detection of salient stimuli and initiate motor responses. For decades, the retina's projections to image-forming centers have served as useful models to elucidate the mechanisms by which such exquisite circuitry is wired. In this chapter, we review the roles of molecular cues, neuronal activity, and axon-axon competition in the development of topographically ordered retinal ganglion cell (RGC) projections to the superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN). Further, we discuss our current state of understanding regarding the laminar-specific targeting of subclasses of RGCs in the SC and its homolog, the optic tectum (OT). Finally, we cover recent studies examining the alignment of projections from primary visual cortex with RGCs that monitor the same region of space in the SC.

Molecular complexity of visual mapping: a challenge for regenerating therapy

Investigating the cellular and molecular mechanisms involved in the development of topographically ordered connections in the central nervous system constitutes an important issue in neurobiology because these connections are the base of the central nervous system normal function. The dominant model to study the development of topographic maps is the projection from the retinal ganglion cells to the optic tectum/colliculus. The expression pattern of Eph/ephrin system in opposing gradients both in the retina and the tectum, labels the local addresses on the target and gives specific sensitivities to growth cones according to their topographic origin in the retina. The rigid precision of normal retinotopic mapping has prompted the chemoaffinity hypothesis, positing axonal targeting to be based on fixed biochemical affinities between fibers and targets. However, several lines of evidence have shown that the mapping can adjust to experimentally modified targets with flexibility, demonstrating the robustness of the guidance process. Here we discuss the complex ways the Ephs and ephrins interact allowing to understand how the retinotectal mapping is a precise but also a flexible process.

Ephrin-A5 potentiates netrin-1 axon guidance by enhancing Neogenin availability

Axonal growth cones are guided by molecular cues in the extracellular environment. The mechanisms of combinatorial integration of guidance signals at the growth cone cell membrane are still being unravelled. Limb-innervating axons of vertebrate spinal lateral motor column (LMC) neurons are attracted to netrin-1 via its receptor, Neogenin, and are repelled from ephrin-A5 through its receptor EphA4. The presence of both cues elicits synergistic guidance of LMC axons, but the mechanism of this effect remains unknown. Using fluorescence immunohistochemistry, we show that ephrin-A5 increases LMC growth cone Neogenin protein levels and netrin-1 binding. This effect is enhanced by overexpressing EphA4 and is inhibited by blocking ephrin-A5-EphA4 binding. These effects have a functional consequence on LMC growth cone responses since bath addition of ephrin-A5 increases the responsiveness of LMC axons to netrin-1. Surprisingly, the overexpression of EphA4 lacking its cytoplasmic tail, also enhances Neogenin levels at the growth cone and potentiates LMC axon preference for growth on netrin-1. Since netrins and ephrins participate in a wide variety of biological processes, the enhancement of netrin-1 signalling by ephrins may have broad implications.

Dynamic Alterations of Retinal EphA5 Expression in Retinocollicular Map Plasticity

The topographically ordered retinocollicular projection is an excellent system for studying the mechanism of axon guidance. Gradients of EphA receptors in the retina and ephrin-As in the superior colliculus (SC) pattern the anteroposterior axis of the retinocollicular map, but whether they are involved in map plasticity after injury is unknown. Partial damage to the caudal SC at birth creates a compressed, complete retinotopic map in the remaining SC without affecting visual response properties. Previously, we found that the gradient of ephrin-A expression in compressed maps is steeper than normal, suggesting an instructive role in compression. Here we measured EphA5 mRNA and protein levels after caudal SC damage in order to test the hypothesis that changes in retinal EphA5 expression occur that are complementary to the changes in collicular ephrin-A expression. We find that the nasotemporal gradient of EphA5 receptor expression steepens in the retina and overall expression levels change dynamically, especially in temporal retina, supporting the hypothesis. This change in receptor expression occurs after the change in ephrin-A ligand expression. We propose that changes in the retinal EphA5 gradient guide recovery of the retinocollicular projection from early injury. This could occur directly through the change in EphA5 expression instructing retino-SC map compression, or through ephrin-A ligand signaling instructing a change in EphA5 receptor expression that in turn signals the retinocollicular map to compress. Understanding what molecular signals direct compensation for injury is essential to developing rehabilitative strategies and maximizing the potential for recovery.

Visual Function, Organization, and Development of the Mouse Superior Colliculus

The superior colliculus (SC) is the most prominent visual center in mice. Studies over the past decade have greatly advanced our understanding of the function, organization, and development of the mouse SC, which has rapidly become a popular model in vision research. These studies have described the diverse and cell-type-specific visual response properties in the mouse SC, revealed their laminar and topographic organizations, and linked the mouse SC and downstream pathways with visually guided behaviors. Here, we summarize these findings, compare them with the rich literature of SC studies in other species, and highlight important gaps and exciting future directions. Given its clear importance in mouse vision and the available modern neuroscience tools, the mouse SC holds great promise for understanding the cellular, circuit, and developmental mechanisms that underlie visual processing, sensorimotor transformation, and, ultimately, behavior.

The Mouse Superior Colliculus: An Emerging Model for Studying Circuit Formation and Function

The superior colliculus (SC) is a midbrain area where visual, auditory and somatosensory information are integrated to initiate motor commands. The SC plays a central role in visual information processing in the mouse; it receives projections from 85% to 90% of the retinal ganglion cells (RGCs). While the mouse SC has been a long-standing model used to study retinotopic map formation, a number of technological advances in mouse molecular genetic techniques, large-scale physiological recordings and SC-dependent visual behavioral assays have made the mouse an even more ideal model to understand the relationship between circuitry and behavior.

Revisiting chemoaffinity theory: Chemotactic implementation of topographic axonal projection

Neural circuits are wired by chemotactic migration of growth cones guided by extracellular guidance cue gradients. How growth cone chemotaxis builds the macroscopic structure of the neural circuit is a fundamental question in neuroscience. I addressed this issue in the case of the ordered axonal projections called topographic maps in the retinotectal system. In the retina and tectum, the erythropoietin-producing hepatocellular (Eph) receptors and their ligands, the ephrins, are expressed in gradients. According to Sperry's chemoaffinity theory, gradients in both the source and target areas enable projecting axons to recognize their proper terminals, but how axons chemotactically decode their destinations is largely unknown. To identify the chemotactic mechanism of topographic mapping, I developed a mathematical model of intracellular signaling in the growth cone that focuses on the growth cone's unique chemotactic property of being attracted or repelled by the same guidance cues in different biological situations. The model presented mechanism by which the retinal growth cone reaches the correct terminal zone in the tectum through alternating chemotactic response between attraction and repulsion around a preferred concentration. The model also provided a unified understanding of the contrasting relationships between receptor expression levels and preferred ligand concentrations in EphA/ephrinA- and EphB/ephrinB-encoded topographic mappings. Thus, this study redefines the chemoaffinity theory in chemotactic terms.

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.

Identification of novel candidate regulators of retinotectal map formation through transcriptional profiling of the chick optic tectum

Information from the retina is carried along the visual pathway with accuracy and spatial conservation as a result of topographically mapped axonal connections. The optic tectum in the midbrain is the primary region to which retinal ganglion cells project their axons in the chick. The two primary axes of the retina project independently onto the tectum using different sets of guidance cues to give rise to the retinotectal map. Specificity of the map is determined by attractive or repulsive interactions between molecular tags that are distributed in gradients in the retina and the tectum. Despite several studies, knowledge of the retinotectal guidance molecules is far from being complete. We screened for all molecules that are expressed differentially along the anterior-posterior and medial-lateral axes of the chick tectum using microarray based transcriptional profiling and identified several novel candidate retinotectal guidance molecules. Two such genes, encoding Wnt5a and Raldh2, the synthesizing enzymes for retinoic acid, were further analyzed for their function as putative regulators of retinotectal map formation. Wnt5a and retinoic acid were found to exhibit differential effects on the growth of axons from retinal explants derived from different quadrants of the retina. This screen also yielded a large number of genes expressed in a lamina-specific manner in the tectum, which may have other roles in tectal development. J. Comp. Neurol. 525:459-477, 2017. © 2016 Wiley Periodicals, Inc.

Developmental Wiring of Specific Neurons Is Regulated by RET-1/Nogo-A in Caenorhabditis elegans

Nogo-A is a membrane-bound protein that functions to inhibit neuronal migration, adhesion, and neurite outgrowth during development. In the mature nervous system, Nogo-A stabilizes neuronal wiring to inhibit neuronal plasticity and regeneration after injury. Here, we show that RET-1, the sole Nogo-A homolog in Caenorhabditis elegans, is required to control developmental wiring of a specific subset of neurons. In ret-1 deletion mutant animals, specific ventral nerve cord axons are misguided where they fail to respect the ventral midline boundary. We found that ret-1 is expressed in multiple neurons during development, and, through mosaic analysis, showed that ret-1 controls axon guidance in a cell-autonomous manner. Finally, as in mammals, ret-1 regulates ephrin expression, and dysregulation of the ephrin ligand VAB-2 is partially responsible for the ret-1 mutant axonal defects. Together, our data present a previously unidentified function for RET-1 in the nervous system of C. elegans.

Ephrin-B/EphB Signaling Is Required for Normal Innervation of Lingual Gustatory Papillae

The innervation of taste buds is an excellent model system for studying the guidance of axons during targeting because of their discrete nature and the high fidelity of innervation. The pregustatory epithelium of fungiform papillae is known to secrete diffusible axon guidance cues such as BDNF and Sema3A that attract and repel, respectively, geniculate ganglion axons during targeting, but diffusible factors alone are unlikely to explain how taste axon terminals are restricted to their territories within the taste bud. Nondiffusible cell surface proteins such as Ephs and ephrins can act as receptors and/or ligands for one another and are known to control axon terminal positioning in several parts of the nervous system, but they have not been studied in the gustatory system. We report that ephrin-B2 linked β-galactosidase staining and immunostaining was present along the dorsal epithelium of the mouse tongue as early as embryonic day 15.5 (E15.5), but was not detected at E14.5, when axons first enter the epithelium. Ephrin-B1 immunolabeling was barely detected in the epithelium and found at a somewhat higher concentration in the mesenchyme subjacent to the epithelium. EphB1 and EphB2 were detected in lingual sensory afferents in vivo and geniculate neurites in vitro. Ephrin-B1 and ephrin-B2 were similarly effective in repelling or suppressing outgrowth by geniculate neurites in vitro. These in vitro effects were independent of the neurotrophin used to promote outgrowth, but were reduced by elevated levels of laminin. In vivo, mice null for EphB1 and EphB2 exhibited decreased gustatory innervation of fungiform papillae. These data provide evidence that ephrin-B forward signaling is necessary for normal gustatory innervation of the mammalian tongue.

The multiple sclerosis drug fingolimod (FTY720) stimulates neuronal gene expression, axonal growth and regeneration

Fingolimod (FTY720) is a new generation oral treatment for multiple sclerosis (MS). So far, FTY720 was mainly considered to target trafficking of immune cells but not brain cells such as neurons. Herein, we analyzed FTY720's potential to directly alter neuronal function. In CNS neurons, we identified a FTY720 governed gene expression response. FTY720 upregulated immediate early genes (IEGs) encoding for neuronal activity associated transcription factors such as c-Fos, FosB, Egr1 and Egr2 and induced actin cytoskeleton associated genes (actin isoforms, tropomyosin, calponin). Stimulation of primary neurons with FTY720 enhanced neurite growth and altered growth cone morphology. In accordance, FTY720 enhanced axon regeneration in mice upon facial nerve axotomy. We identified components of a FTY720 engaged signaling cascade including S1P receptors, G12/13G-proteins, RhoA-GTPases and the transcription factors SRF/MRTF. In summary, we uncovered a broader cellular and therapeutic operation mode of FTY720, suggesting beneficial FTY720 effects also on CNS neurons during MS therapy and for treatment of other neurodegenerative diseases requiring neuroprotective and neurorestorative processes.

Mechanisms of ephrin-Eph signalling in development, physiology and disease

Eph receptor Tyr kinases and their membrane-tethered ligands, the ephrins, elicit short-distance cell-cell signalling and thus regulate many developmental processes at the interface between pattern formation and morphogenesis, including cell sorting and positioning, and the formation of segmented structures and ordered neural maps. Their roles extend into adulthood, when ephrin-Eph signalling regulates neuronal plasticity, homeostatic events and disease processes. Recently, new insights have been gained into the mechanisms of ephrin-Eph signalling in different cell types, and into the physiological importance of ephrin-Eph in different organs and in disease, raising questions for future research directions.

Draxin from neocortical neurons controls the guidance of thalamocortical projections into the neocortex

The thalamocortical tract carries sensory information to the neocortex. It has long been recognized that the neocortical pioneer axons of subplate neurons are essential for thalamocortical development. Herein we report that an axon guidance cue, draxin, is expressed in early-born neocortical neurons, including subplate neurons, and is necessary for thalamocortical development. In draxin^−/− mice, thalamocortical axons do not enter the neocortex. This phenotype is sufficiently rescued by the transgenic expression of draxin in neocortical neurons. Genetic interaction data suggest that draxin acts through Deleted in colorectal cancer (DCC) and Neogenin (Neo1), to regulate thalamocortical projections in vivo. Draxin promotes the outgrowth of thalamic axons in vitro and this effect is abolished in thalamic neurons from Dcc and Neo1 double mutants. These results suggest that draxin from neocortical neurons controls thalamocortical projections into the neocortex, and that this effect is mediated through the DCC and Neo1 receptors.

During neural development thalamocortical axons follow corticofugal projections into the neocortex. Here, using a combination of knock down and rescue experiments, the authors show that Draxin expression in neocortical cells promotes thalamic axon projections from the internal capsule.

Rapid Changes in Cortical and Subcortical Brain Regions after Early Bilateral Enucleation in the Mouse

Functional sensory and motor areas in the developing mammalian neocortex are formed through a complex interaction of cortically intrinsic mechanisms, such as gene expression, and cortically extrinsic mechanisms such as those mediated by thalamic input from the senses. Both intrinsic and extrinsic mechanisms are believed to be involved in cortical patterning and the establishment of areal boundaries in early development; however, the nature of the interaction between intrinsic and extrinsic processes is not well understood. In a previous study, we used a perinatal bilateral enucleation mouse model to test some aspects of this interaction by reweighting sensory input to the developing cortex. Visual deprivation at birth resulted in a shift of intraneocortical connections (INCs) that aligned with ectopic ephrin A5 expression in the same location ten days later at postnatal day (P) 10. A prevailing question remained: Does visual deprivation first induce a change in gene expression, followed by a shift in INCs, or vice versa? In the present study, we address this question by investigating the neuroanatomy and patterns of gene expression in post-natal day (P) 1 and 4 mice following bilateral enucleation at birth. Our results demonstrate a rapid reduction in dorsal lateral geniculate nucleus (dLGN) size and ephrin A5 gene expression 24-hours post-enucleation, with more profound effects apparent at P4. The reduced nuclear size and diminished gene expression mirrors subtle changes in ephrin A5 expression evident in P1 and P4 enucleated neocortex, 11 and 8 days prior to natural eye opening, respectively. Somatosensory and visual INCs were indistinguishable between P1 and P4 mice bilaterally enucleated at birth, indicating that perinatal bilateral enucleation initiates a rapid change in gene expression (within one day) followed by an alteration of sensory INCs later on (second postnatal week). With these results, we gain a deeper understanding of how gene expression and sensory input together regulate cortical arealization and plasticity during early development.

EphA7 regulates spiral ganglion innervation of cochlear hair cells

During the development of periphery auditory circuitry, spiral ganglion neurons (SGNs) form a spatially precise pattern of innervation of cochlear hair cells (HCs), which is an essential structural foundation for central auditory processing. However, molecular mechanisms underlying the developmental formation of this precise innervation pattern remain not well understood. Here, we specifically examined the involvement of Eph family members in cochlear development. By performing RNA-sequencing for different types of cochlear cell, in situ hybridization, and immunohistochemistry, we found that EphA7 was strongly expressed in a large subset of SGNs. In EphA7 deletion mice, there was a reduction in the number of inner radial bundles originating from SGNs and projecting to HCs as well as in the number of ribbon synapses on inner hair cells (IHCs), as compared with wild-type or heterozygous mutant mice, attributable to fewer type I afferent fibers. The overall activity of the auditory nerve in EphA7 deletion mice was also reduced, although there was no significant change in the hearing intensity threshold. In vitro analysis further suggested that the reduced innervation of HCs by SGNs could be attributed to a role of EphA7 in regulating outgrowth of SGN neurites as knocking down EphA7 in SGNs resulted in diminished SGN fibers. In addition, suppressing the activity of ERK1/2, a potential downstream target of EphA7 signaling, either with specific inhibitors in cultured explants or by knocking out Prkg1, also resulted in reduced SGN fibers. Together, our results suggest that EphA7 plays an important role in the developmental formation of cochlear innervation pattern through controlling SGN fiber ontogeny. Such regulation may contribute to the salience level of auditory signals presented to the central auditory system.

Mechanisms of boundary formation by Eph receptor and ephrin signaling

The formation of sharp borders, across which cell intermingling is restricted, has a crucial role in the establishment and maintenance of organized tissues. Signaling of Eph receptors and ephrins underlies formation of a number of boundaries between and within tissues during vertebrate development. Eph-ephrin signaling can regulate several types of cell response-adhesion, repulsion and tension-that can in principle underlie the segregation of cells and formation of sharp borders. Recent studies have implicated each of these cell responses as having important roles at different boundaries: repulsion at the mesoderm-ectoderm border, decreased adhesion at the notochord-presomitic mesoderm border, and tension at boundaries within the hindbrain and forebrain. These distinct responses to Eph receptor and ephrin activation may in part be due to the adhesive properties of the tissue.

Quantitative assessment of computational models for retinotopic map formation

Molecular and activity-based cues acting together are thought to guide retinal axons to their terminal sites in vertebrate optic tectum or superior colliculus (SC) to form an ordered map of connections. The details of mechanisms involved, and the degree to which they might interact, are still not well understood. We have developed a framework within which existing computational models can be assessed in an unbiased and quantitative manner against a set of experimental data curated from the mouse retinocollicular system. Our framework facilitates comparison between models, testing new models against known phenotypes and simulating new phenotypes in existing models. We have used this framework to assess four representative models that combine Eph/ephrin gradients and/or activity-based mechanisms and competition. Two of the models were updated from their original form to fit into our framework. The models were tested against five different phenotypes: wild type, Isl2-EphA3(ki/ki), Isl2-EphA3(ki/+), ephrin-A2,A3,A5 triple knock-out (TKO), and Math5(-/-) (Atoh7). Two models successfully reproduced the extent of the Math5(-/-) anteromedial projection, but only one of those could account for the collapse point in Isl2-EphA3(ki/+). The models needed a weak anteroposterior gradient in the SC to reproduce the residual order in the ephrin-A2,A3,A5 TKO phenotype, suggesting either an incomplete knock-out or the presence of another guidance molecule. Our article demonstrates the importance of testing retinotopic models against as full a range of phenotypes as possible, and we have made available MATLAB software, we wrote to facilitate this process.

Target-independent ephrina/EphA-mediated axon-axon repulsion as a novel element in retinocollicular mapping

EphrinAs and EphAs play critical roles during topographic map formation in the retinocollicular projection; however, their complex expression patterns in both the retina and superior colliculus (SC) have made it difficult to uncover their precise mechanisms of action. We demonstrate here that growth cones of temporal axons collapse when contacting nasal axons in vitro, and removing ephrinAs from axonal membranes by PI-PLC treatment abolishes this response. In conditional knockout mice, temporal axons display no major targeting defects when ephrinA5 is removed only from the SC, but substantial mapping defects were observed when ephrinA5 expression was removed from both the SC and from the retina, with temporal axons invading the target areas of nasal axons. Together, these data indicate that ephrinA5 drives repellent interactions between temporal and nasal axons within the SC, and demonstrates for the first time that target-independent mechanisms play an essential role in retinocollicular map formation in vivo.

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EphrinA expression on nasal axons mediates growth cone collapse of temporal axons

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Analysis of retinocollicular projection in a conditional knockout of ephrinA5

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Disruption of repellent axon-axon interactions leads to mapping defects

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In vivo evidence for target-independent mapping mechanisms in visual system

Computational and mathematical methods for morphogenetic gradient analysis, boundary formation and axonal targeting

Morphogenesis and axonal targeting are key processes during development that depend on complex interactions at molecular, cellular and tissue level. Mathematical modeling is essential to bridge this multi-scale gap in order to understand how the emergence of large structures is controlled at molecular level by interactions between various signaling pathways. We summarize mathematical modeling and computational methods for time evolution and precision of morphogenetic gradient formation. We discuss tissue patterning and the formation of borders between regions labeled by different morphogens. Finally, we review models and algorithms that reveal the interplay between morphogenetic gradients and patterned activity for axonal pathfinding and the generation of the retinotopic map in the visual system.

Chemoaffinity in topographic mapping revisited--is it more about fiber-fiber than fiber-target interactions?

Axonal projections between two populations of neurons, which preserve neighborhood relationships, are called topographic. They are ubiquitous in the brain. The development of the retinotectal projection, mapping the retinal output onto the roof of the midbrain, has been studied for decades as a model system. The rigid precision of normal retinotopic mapping has prompted the chemoaffinity hypothesis, positing axonal targeting to be based on fixed biochemical affinities between fibers and targets. In addition, however, abundant evidence has been gathered mainly in the 1970s and 80s that the mapping can adjust to variegated targets with stunning flexibility demonstrating the extraordinary robustness of the guidance process. The identification of ephrins and Eph-receptors as the underlying molecular cues has mostly been interpreted as supporting the fiber-target chemoaffinity hypothesis, while the evidence on mapping robustness has largely been neglected. By having a fresh look on the old data, we expound that they indicate, in addition to fiber-target chemoaffinity, the existence of a second autonomous guidance influence, which we call fiber-fiber chemoaffinity. Classical in vitro observations suggest both influences be composed of opposing monofunctional guidance activities. Based on the molecular evidence, we propose that those might be ephrin/Eph forward and reverse signaling, not only in fiber-target but also in fiber-fiber interactions. In fact, computational models based on this assumption can reconcile the seemingly conflicting findings on rigid and flexible topographic mapping. Supporting the suggested parsimonious and powerful mechanism, they contribute to an understanding of the evolutionary success of robust topographic mass wiring of axons.

Parcellation of the thalamus into distinct nuclei reflects EphA expression and function

Intercellular signaling via the Eph receptor tyrosine kinases and their ligands, the ephrins, acts to shape many regions of the developing brain. One intriguing consequence of Eph signaling is the control of mixing between discrete cell populations in the developing hindbrain, contributing to the formation of segregated rhombomeres. Since the thalamus is also a parcellated structure comprised of discrete nuclei, might Eph signaling play a parallel role in cell segregation in this brain structure? Analyses of expression reveal that several Eph family members are expressed in the forming thalamus and that cells expressing particular receptors form cellular groupings as development proceeds. Specifically, expression of receptors EphA4 or EphA7 and ligand ephrin-A5 is localized to distinct thalamic domains. EphA4 and EphA7 are often coexpressed in regions of the forming thalamus, with each receptor marking discrete thalamic domains. In contrast, ephrin-A5 is expressed by a limited group of thalamic cells. Within the ventral thalamus, EphA4 is present broadly, occasionally overlapping with ephrin-A5 expression. EphA7 is more restricted in its expression and is largely nonoverlapping with ephrin-A5. In mutant mice lacking one or both receptors or ephrin-A5, the appearance of the venteroposterolateral (VPL) and venteroposteromedial (VPM) nuclear complex is altered compared to wild type mice. These in vivo results support a role for Eph family members in the definition of the thalamic nuclei. In parallel, in vitro analysis reveals a hierarchy of mixing among cells expressing ephrin-A5 with cells expressing EphA4 alone, EphA4 and EphA7 together, or EphA7 alone. Together, these data support a model in which EphA molecules promote the parcellation of discrete thalamic nuclei by limiting the extent of cell mixing.

Nel positively regulates the genesis of retinal ganglion cells by promoting their differentiation and survival during development

For correct functioning of the nervous system, the appropriate number and complement of neuronal cell types must be produced during development. However, the molecular mechanisms that regulate the production of individual classes of neurons are poorly understood. In this study, we investigate the function of the thrombospondin-1-like glycoprotein, Nel (neural epidermal growth factor [EGF]-like), in the generation of retinal ganglion cells (RGCs) in chicks. During eye development, Nel is strongly expressed in the presumptive retinal pigment epithelium and RGCs. Nel overexpression in the developing retina by in ovo electroporation increases the number of RGCs, whereas the number of displaced amacrine cells decreases. Conversely, knockdown of Nel expression by transposon-mediated introduction of RNA interference constructs results in decrease in RGC number and increase in the number of displaced amacrine cells. Modifications of Nel expression levels do not appear to affect proliferation of retinal progenitor cells, but they significantly alter the progression rate of RGC differentiation from the central retina to the periphery. Furthermore, Nel protects RGCs from apoptosis during retinal development. These results indicate that Nel positively regulates RGC production by promoting their differentiation and survival during development.

Mechanisms of ephrin receptor protein kinase-independent signaling in amphid axon guidance in Caenorhabditis elegans

Eph receptors and their ephrin ligands are key conserved regulators of axon guidance and can function in a variety of signaling modes. Here we analyze the genetic and cellular requirements for Eph signaling in a Caenorhabditis elegans axon guidance choice point, the ventral guidance of axons in the amphid commissure. The C. elegans Eph receptor EFN-1 has both kinase-dependent and kinase-independent roles in amphid ventral guidance. Of the four C. elegans ephrins, we find that only EFN-1 has a major role in amphid axon ventral guidance, and signals in both a receptor kinase-dependent and kinase-independent manner. Analysis of EFN-1 and EFN-1 expression and tissue-specific requirements is consistent with a model in which VAB-1 acts in amphid neurons, interacting with EFN-1 expressed on surrounding cells. Unexpectedly, left-hand neurons are more strongly affected than right-hand neurons by loss of Eph signaling, indicating a previously undetected left-right asymmetry in the requirement for Eph signaling. By screening candidate genes involved in Eph signaling, we find that the Eph kinase-independent pathway involves the ABL-1 nonreceptor tyrosine kinase and possibly the phosphatidylinositol 3-kinase pathway. Overexpression of ABL-1 is sufficient to rescue EFN-1 ventral guidance defects cell autonomously. Our results reveal new aspects of Eph signaling in a single axon guidance decision in vivo.

Molecular guidance of retinotopic map development in the midbrain

Topographic maps are utilized in many sensory and motor systems to efficiently transfer information between brain regions. The retina's projection to the superior colliculus has served as a model for the identification of molecular cues and mechanistic strategies by which topographic maps are formed. Evidence from both in vitro and in vivo studies points to graded cell surface cues playing a central role, but support for axon-axon competition and selective degeneration have also been advanced recently. In combination with mathematical models, these studies suggest that topographic maps are established using a complex combination of strategies to ensure precise connectivity.

Simultaneous generation of gradients with gradually changed slope in a microfluidic device for quantifying axon response

Over the past decades, various microfluidic devices have been developed to investigate the role of the molecular gradient in axonal development; however, there are very few devices providing quantitative information about the response of axons to molecular gradients with different slopes. Here, we propose a novel laminar-based microfluidic device enabling simultaneous generation of multiple gradients with gradually changed slope on a single chip. This device, with two asymmetrically designed peripheral channels and opposite flow direction, could generate gradients with gradually changed slope in the center channel, enabling us to investigate simultaneously the response of axons to multiple slope gradients with the same batch of neurons. We quantitatively investigated the response of axon growth rate and growth direction to substrate-bound laminin gradients with different slopes using this single-layer chip. Furthermore, we compartmented this gradient generation chip and a cell culture chip by a porous membrane to investigate quantitatively the response of axon growth rate to the gradient of soluble factor netrin-1. The results suggested that contacting with a molecular gradient would effectively accelerate neurites growth and enhance axonal formation, and the axon guidance ratio obviously increased with the increase of gradient slope in a proper range. The capability of generating a molecular gradient with continuously variable slopes on a single chip would open up opportunities for obtaining quantitative information about the sensitivity of axons and other types of cells in response to gradients of various proteins.

Structural and molecular micropatterning of dual hydrogel constructs for neural growth models using photochemical strategies

Chemotactic and haptotactic cues guide neurite growth toward appropriate targets by eliciting attractive or repulsive responses from the neurite growth cones. Here we present an integrated system allowing both structural and molecular micropatterning in dual hydrogel 3D tissue culture constructs for directing in vitro neuronal growth via structural, immobilized, and soluble guidance cues. These tissue culture constructs were fabricated into specifiable geometries using UV light reflected from a digital micromirror device acting as a dynamic photomask, resulting in dual hydrogel constructs consisting of a cell growth-restrictive polyethylene glycol (PEG) boundary with a cell growth-permissive interior of photolabile α-carboxy-2-nitrobenzyl cysteine agarose (CNBC-A). This CNBC-A was irradiated in discrete areas and subsequently tagged with maleimide-conjugated biomolecules. Fluorescent microscopy showed biomolecule binding only at the sites of irradiation in CNBC-A, and confocal microscopy confirmed 3D binding through the depth of the construct. Neurite outgrowth studies showed contained growth throughout CNBC-A. The diffusion rate of soluble fluorescein-bovine serum albumin through the dual hydrogel construct was controlled by PEG concentration and the distance between the protein source and the agarose interior; the timescale for a transient protein gradient changed with these parameters. These findings suggest the dual hydrogel system is a useful platform for manipulating a 3D in vitro microenvironment with patterned structural and molecular guidance cues for modeling neural growth and guidance.

Role of EphA/ephrin--a signaling in the development of topographic maps in mouse corticothalamic projections

Corticothalamic (CT) feedback outnumbers thalamocortical projections and regulates sensory information processing at the level of the thalamus. It is well established that EphA7, a member of EphA receptor family, is involved in the topographic mapping of CT projections. The aim of the present study was to dissect the precise impact of EphA7 on each step of CT growth. We used in utero electroporation-mediated EphA7 overexpression in developing somatosensory CT axons to dissect EphA7/ephrin-A-dependent mechanisms involved in regulating both initial targeting and postnatal growth of the CT projections. Our data revealed that topographic maps of cortical afferents in the ventrobasal complex and medial part of the posterior complex in the thalamus become discernible shortly after birth and are fully established by the second postnatal week. This process starts with the direct ingrowth of the CT axons to the designated areas within target thalamic nuclei and by progressive increase of axonal processes in the terminal zones. Large-scale overproduction and elimination of exuberant widespread axonal branches outside the target zone was not observed. Each developmental event was coordinated by spatially and temporally different responsiveness of CT axons to the ephrin-A gradient in thalamic nuclei, as well as by the matching levels of EphA7 in CT axons and ephrin-As in thalamic nuclei. These results support the concept that the topographic connections between the maps in the cerebral cortex and corresponding thalamic nuclei are genetically prespecified to a large extent, and established by precise spatiotemporal molecular mechanisms that involve the Eph family of genes.

Sensory neuron-derived eph regulates glomerular arbors and modulatory function of a central serotonergic neuron

Olfactory sensory neurons connect to the antennal lobe of the fly to create the primary units for processing odor cues, the glomeruli. Unique amongst antennal-lobe neurons is an identified wide-field serotonergic neuron, the contralaterally-projecting, serotonin-immunoreactive deutocerebral neuron (CSDn). The CSDn spreads its termini all over the contralateral antennal lobe, suggesting a diffuse neuromodulatory role. A closer examination, however, reveals a restricted pattern of the CSDn arborization in some glomeruli. We show that sensory neuron-derived Eph interacts with Ephrin in the CSDn, to regulate these arborizations. Behavioural analysis of animals with altered Eph-ephrin signaling and with consequent arborization defects suggests that neuromodulation requires local glomerular-specific patterning of the CSDn termini. Our results show the importance of developmental regulation of terminal arborization of even the diffuse modulatory neurons to allow them to route sensory-inputs according to the behavioural contexts.

Serotonin, a major neuromodulatory transmitter, regulates diverse behaviours. Serotonergic dysfunction is implicated in various neuropsychological disorders, such as anxiety and depression, as well as in neurodegenerative disorders. In the central nervous systems, across taxa, serotonergic neurons are often small in number but connect to and act upon multiple brain circuits through their wide-field arborization pattern. We set out to decipher mechanisms by which wide-field serotonergic neurons differentially innervate their target-field to modulate behavior in a context-dependent manner. We took advantage of the sophisticated antennal lobe circuitry, the primary olfactory centre in the adult fruitfly Drosophila melanogaster. Olfactory sensory neurons and projection neurons connect in a partner-specific manner to create glomerular units in the antennal lobe for processing the sense of smell. Our analysis at a single-cell resolution reveals that a wide-field serotonergic neuron connects to all the glomeruli in the antennal lobe but exhibits the glomerular-specific differences in its innervation pattern. Our key finding is that Eph from sensory neurons regulates the glomerular-specific innervation pattern of the central serotonergic neuron, which in turn is essential for modulation of odor-guided behaviours in an odor-specific manner.

Integration of guidance cues: parallel signaling and crosstalk

Growing axons are exposed to various guidance cues en route to their targets. Although many guidance molecules have been identified and their effects on axon behavior extensively studied, how axons react to combinations of signals remains largely unexplored. We review recent studies investigating the combined actions of guidance cues present at the same choice points. Two main scenarios are emerging from these studies: parallel signaling and crosstalk between guidance systems. In the first case, cues act in an additive manner, whereas in the second case the outcome is non-additive and differs from the sum of individual effects, suggesting more complex signal integration in the growth cone. Some of the molecular mechanisms underlying these interactions are beginning to be unraveled.

Boundary formation in the development of the vertebrate hindbrain

The formation of a sharp interface of adjacent subdivisions is important for establishing the precision of tissue organization, and at specific borders it serves to organize key signaling centers. We discuss studies of vertebrate hindbrain development that have given important insights into mechanisms that underlie the formation and maintenance of sharp borders. The hindbrain is subdivided into a series of segments with distinct anteroposterior identity that underlies the specification of distinct neuronal cell types. During early stages of segmentation, cell identity switching contributes to the refinement of borders and enables homogenous territories to be maintained despite intermingling of cells between segments. At later stages, there is a specific restriction to cell intermingling between segments that is mediated by Eph receptor and ephrin signaling. Eph-ephrin signaling can restrict cell intermingling and sharpen borders through multiple mechanisms, including the regulation of cell adhesion and contact inhibition of cell migration.

Eph receptor signaling in C. elegans

Eph receptor protein-tyrosine kinases are among the oldest known animal receptors and have greatly expanded in number during vertebrate evolution. Their complex transduction mechanisms are capable of bidirectional and bimodal (multi-response) signaling. Eph receptors are expressed in almost every cell type in the human body, yet their roles in development, physiology, and disease are incompletely understood. Studies in C. elegans have helped identify biological functions of these receptors, as well as transduction mechanisms. Here we review advances in our understanding of Eph receptor signaling made using the C. elegans model system.

Regulation of ephrin-A expression in compressed retinocollicular maps

Retinotopic maps can undergo compression and expansion in response to changes in target size, but the mechanism underlying this compensatory process has remained a mystery. The discovery of ephrins as molecular mediators of Sperry's chemoaffinity process allows a mechanistic approach to this important issue. In Syrian hamsters, neonatal, partial (PT) ablation of posterior superior colliculus (SC) leads to compression of the retinotopic map, independent of neural activity. Graded, repulsive EphA receptor/ephrin-A ligand interactions direct the formation of the retinocollicular map, but whether ephrins might also be involved in map compression is unknown. To examine whether map compression might be directed by changes in the ephrin expression pattern, we compared ephrin-A2 and ephrin-A5 mRNA expression between normal SC and PT SC using in situ hybridization and quantitative real-time PCR. We found that ephrin-A ligand expression in the compressed maps was low anteriorly and high posteriorly, as in normal animals. Consistent with our hypothesis, the steepness of the ephrin gradient increased in the lesioned colliculi. Interestingly, overall levels of ephrin-A2 and -A5 expression declined immediately after neonatal target damage, perhaps promoting axon outgrowth. These data establish a correlation between changes in ephrin-A gradients and map compression, and suggest that ephrin-A expression gradients may be regulated by target size. This in turn could lead to compression of the retinocollicular map onto the reduced target. These findings have important implications for mechanisms of recovery from traumatic brain injury.

Differing semaphorin 3A concentrations trigger distinct signaling mechanisms in growth cone collapse

Semaphorin-3A (Sema3A) is a major guidance cue in the developing nervous system. Previous studies have revealed a dependence of responses to Sema3A on local protein synthesis (PS) in axonal growth cones, but a recent study has called this dependence into question. To understand the basis of this discrepancy we used the growth cone collapse assay on chick dorsal root ganglion neurons. We show that the dependence of growth cone collapse on protein synthesis varies according to Sema3A concentration, from near-total at low concentration (<100 ng/ml) to minimal at high concentration (>625 ng/ml). Further, we show that neuropilin-1 (NP-1) mediates both PS-dependent and PS-independent collapse. Our findings are consistent with the operation of at least two distinct Sema3A signaling pathways: one that is PS-dependent, involving mammalian target of rapamycin, and one that is PS-independent, involving GSK-3β activation and operative at all concentrations of Sema3A examined. The results provide a plausible explanation for the discrepancy in PS-dependence reported in the literature, and indicate that different signaling pathways activated within growth cones can be modulated by changing the concentration of the same guidance cue.

'Til Eph do us part': intercellular signaling via Eph receptors and ephrin ligands guides cerebral cortical development from birth through maturation

Eph receptors, the largest family of surface-bound receptor tyrosine kinases and their ligands, the ephrins, mediate a wide variety of cellular interactions in most organ systems throughout both development and maturity. In the forming cerebral cortex, Eph family members are broadly and dynamically expressed in particular sets of cortical cells at discrete times. Here, we review the known functions of Eph-mediated intercellular signaling in the generation of progenitors, the migration of maturing cells, the differentiation of neurons, the formation of functional connections, and the choice between life and death during corticogenesis. In synthesizing these results, we posit a signaling paradigm in which cortical cells maintain a life history of Eph-mediated intercellular interactions that guides subsequent cellular decision-making.

EphB signaling regulates target innervation in the developing and deafferented auditory brainstem

Precision in auditory brainstem connectivity underlies sound localization. Cochlear activity is transmitted to the ventral cochlear nucleus (VCN) in the mammalian brainstem via the auditory nerve. VCN globular bushy cells project to the contralateral medial nucleus of the trapezoid body (MNTB), where specialized axons terminals, the calyces of Held, encapsulate MNTB principal neurons. The VCN-MNTB pathway is an essential component of the circuitry used to compute interaural intensity differences that are used for localizing sounds. When input from one ear is removed during early postnatal development, auditory brainstem circuitry displays robust anatomical plasticity. The molecular mechanisms that control the development of auditory brainstem circuitry and the developmental plasticity of these pathways are poorly understood. In this study we examined the role of EphB signaling in the development of the VCN-MNTB projection and in the reorganization of this pathway after unilateral deafferentation. We found that EphB2 and EphB3 reverse signaling are critical for the normal development of the projection from VCN to MNTB, but that successful circuit assembly most likely relies upon the coordinated function of many EphB proteins. We have also found that ephrin-B reverse signaling repels induced projections to the ipsilateral MNTB after unilateral deafferentation, suggesting that similar mechanisms regulate these two processes.

Eph-dependent cell-cell adhesion and segregation in development and cancer

Numerous studies attest to essential roles for Eph receptors and their ephrin ligands in controlling cell positioning and tissue patterning during normal and oncogenic development. These studies suggest multiple, sometimes contradictory, functions of Eph-ephrin signalling, which under different conditions can promote either spreading and cell-cell adhesion or cytoskeletal collapse, cell rounding, de-adhesion and cell-cell segregation. A principle determinant of the balance between these two opposing responses is the degree of receptor/ligand clustering and activation. This equilibrium is likely altered in cancers and modulated by somatic mutations of key Eph family members that have emerged as candidate cancer markers in recent profiling studies. In addition, cross-talk amongst Ephs and with other signalling pathways significantly modulates cell-cell adhesion, both between and within Eph- and ephrin-expressing cell populations. This review summarises our current understanding of how Eph receptors control cell adhesion and morphology, and presents examples demonstrating the importance of these events in normal development and cancer.

A novel feedback mechanism by Ephrin-B1/B2 in T-cell activation involves a concentration-dependent switch from costimulation to inhibition

Bidirectional signals via Eph receptors/ephrins have been recognized as major forms of contact-dependent cell communications such as cell attraction and repulsion. T cells express EphBs, and their ligands, the ephrin-Bs, have been known as costimulatory molecules for T-cell proliferation. Recently, another remarkable feature of ephrin-As has emerged in the form of a concentration-dependent transition from promotion to inhibition in axon growth. Here we examined whether this modification plays a role in ephrin-B costimulation in murine primary T cells. Low doses of ephrin-B1 and ephrin-B2 costimulated T-cell proliferation induced by anti-CD3, but high concentrations strongly inhibited it. In contrast, ephrin-B3 showed a steadily increasing stimulatory effect. This modulation was virtually preserved in T cells from mice simultaneously lacking four genes, EphB1, EphB2, EphB3, and EphB6. High concentrations of ephrin-B1/B2, but not ephrin-B3, inhibited the anti-CD3-induced phosphorylation of Lck and its downstream signals such as Erk and Akt. Additionally, high doses of any ephrin-Bs could phosphorylate EphB4. However, only ephrin-B1/B2 but not ephrin-B3 recruited SHP1, a phosphatase to suppress the phosphorylation of Lck. These data suggest that EphB4 signaling could engage in negative feedback to TCR signals. T-cell activation may be finely adjusted by the combination and concentration of ephrin-Bs expressed in the immunological microenvironment.

EphB regulates L1 phosphorylation during retinocollicular mapping

Interaction of the cell adhesion molecule L1 with the cytoskeletal adaptor ankyrin is essential for topographic mapping of retinal ganglion cell (RGC) axons to synaptic targets in the superior colliculus (SC). Mice mutated in the L1 ankyrin-binding motif (FIGQY(1229)H) display abnormal mapping of RGC axons along the mediolateral axis of the SC, resembling mouse mutant phenotypes in EphB receptor tyrosine kinases. To investigate whether L1 functionally interacts with EphBs, we investigated the role of EphB kinases in phosphorylating L1 using a phospho-specific antibody to the tyrosine phosphorylated FIGQY(1229) motif. EphB2, but not an EphB2 kinase dead mutant, induced tyrosine phosphorylation of L1 at FIGQY(1229) and perturbed ankyrin recruitment to the membrane in L1-transfected HEK293 cells. Src family kinases mediated L1 phosphorylation at FIGQY(1229) by EphB2. Other EphB receptors that regulate medial-lateral retinocollicular mapping, EphB1 and EphB3, also mediated phosphorylation of L1 at FIGQY(1229). Tyrosine(1176) in the cytoplasmic domain of L1, which regulates AP2/clathrin-mediated endocytosis and axonal trafficking, was not phosphorylated by EphB2. Accordingly mutation of Tyr(1176) to Ala in L1-Y(1176)A knock-in mice resulted in normal retinocollicular mapping of ventral RGC axons. Immunostaining of the mouse SC during retinotopic mapping showed that L1 colocalized with phospho-FIGQY in RGC axons in retinorecipient layers. Immunoblotting of SC lysates confirmed that L1 was phosphorylated at FIGQY(1229) in wild type but not L1-FIGQY(1229)H (L1Y(1229)H) mutant SC, and that L1 phosphorylation was decreased in the EphB2/B3 mutant SC. Inhibition of ankyrin binding in L1Y(1229)H mutant RGCs resulted in increased neurite outgrowth compared to WT RGCs in retinal explant cultures, suggesting that L1-ankyrin binding serves to constrain RGC axon growth. These findings are consistent with a model in which EphB kinases phosphorylate L1 at FIGQY(1229) in retinal axons to modulate L1-ankyrin binding important for mediolateral retinocollicular topography.