Retinal axon growth cones respond to EphB extracellular domains as inhibitory axon guidance cues

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.

Signaling - transcription interactions in mouse retinal ganglion cells early axon pathfinding -a literature review

Sending an axon out of the eye and into the target brain nuclei is the defining feature of retinal ganglion cells (RGCs). The literature on RGC axon pathfinding is vast, but it focuses mostly on decision making events such as midline crossing at the optic chiasm or retinotopic mapping at the target nuclei. In comparison, the exit of RGC axons out of the eye is much less explored. The first checkpoint on the RGC axons' path is the optic cup - optic stalk junction (OC-OS). OC-OS development and the exit of the RGC pioneer axons out of the eye are coordinated spatially and temporally. By the time the optic nerve head domain is specified, the optic fissure margins are in contact and the fusion process is ongoing, the first RGCs are born in its proximity and send pioneer axons in the optic stalk. RGC differentiation continues in centrifugal waves. Later born RGC axons fasciculate with the more mature axons. Growth cones at the end of the axons respond to guidance cues to adopt a centripetal direction, maintain nerve fiber layer restriction and to leave the optic cup. Although there is extensive information on OC-OS development, we still have important unanswered questions regarding its contribution to the exit of the RGC axons out of the eye. We are still to distinguish the morphogens of the OC-OS from the axon guidance molecules which are expressed in the same place at the same time. The early RGC transcription programs responsible for axon emergence and pathfinding are also unknown. This review summarizes the molecular mechanisms for early RGC axon guidance by contextualizing mouse knock-out studies on OC-OS development with the recent transcriptomic studies on developing RGCs in an attempt to contribute to the understanding of human optic nerve developmental anomalies. The published data summarized here suggests that the developing optic nerve head provides a physical channel (the closing optic fissure) as well as molecular guidance cues for the pioneer RGC axons to exit the eye.

Cannabigerol Activates Cytoskeletal Remodeling via Wnt/PCP in NSC-34: An In Vitro Transcriptional Study

Cannabigerol (CBG) is a non-psychoactive phytocannabinoid present in the Cannabis sativa L. plant. In our study, CBG at the concentration of 10 µM was used to treat NSC-34 motor neuron-like cells. The aim of the study was to evaluate the effects of CBG on NSC-34 cells, using next-generation sequencing (NGS) technology. Analysis showed the activation of the WNT/planar cell polarity (PCP) pathway and Ephrin-Eph signaling. The results revealed that CBG increases the expression of genes associated with the onset process of cytoskeletal remodeling and axon guidance.

Understanding the development of oral epithelial organs through single cell transcriptomic analysis

During craniofacial development, the oral epithelium begins as a morphologically homogeneous tissue that gives rise to locally complex structures, including the teeth, salivary glands and taste buds. How the epithelium is initially patterned and specified to generate diverse cell types remains largely unknown. To elucidate the genetic programs that direct the formation of distinct oral epithelial populations, we mapped the transcriptional landscape of embryonic day 12 mouse mandibular epithelia at single cell resolution. Our analysis identified key transcription factors and gene regulatory networks that define different epithelial cell types. By examining the spatiotemporal patterning process along the oral-aboral axis, our results propose a model in which the dental field is progressively confined to its position by the formation of the aboral epithelium anteriorly and the non-dental oral epithelium posteriorly. Using our data, we also identified Ntrk2 as a proliferation driver in the forming incisor, contributing to its invagination. Together, our results provide a detailed transcriptional atlas of the embryonic mandibular epithelium, and unveil new genetic markers and regulators that are present during the specification of various oral epithelial structures.

A Subtle Network Mediating Axon Guidance: Intrinsic Dynamic Structure of Growth Cone, Attractive and Repulsive Molecular Cues, and the Intermediate Role of Signaling Pathways

A fundamental feature of both early nervous system development and axon regeneration is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. In the navigation process where the nerves grow toward their targets, the growth cones, which locate at the tips of axons, sense the environment surrounding them, including varies of attractive or repulsive molecular cues, then make directional decisions to adjust their navigation journey. The turning ability of a growth cone largely depends on its highly dynamic skeleton, where actin filaments and microtubules play a very important role in its motility. In this review, we summarize some possible mechanisms underlying growth cone motility, relevant molecular cues, and signaling pathways in axon guidance of previous studies and discuss some questions regarding directions for further studies.

Motor axon exit from the mammalian spinal cord is controlled by the homeodomain protein Nkx2.9 via Robo-Slit signaling

Mammalian motor circuits control voluntary movements by transmitting signals from the central nervous system (CNS) to muscle targets. To form these circuits, motor neurons (MNs) must extend their axons out of the CNS. Although exit from the CNS is an indispensable phase of motor axon pathfinding, the underlying molecular mechanisms remain obscure. Here, we present the first identification of a genetic pathway that regulates motor axon exit from the vertebrate spinal cord, utilizing spinal accessory motor neurons (SACMNs) as a model system. SACMNs are a homogeneous population of spinal MNs with axons that leave the CNS through a discrete lateral exit point (LEP) and can be visualized by the expression of the cell surface protein BEN. We show that the homeodomain transcription factor Nkx2.9 is selectively required for SACMN axon exit and identify the Robo2 guidance receptor as a likely downstream effector of Nkx2.9; loss of Nkx2.9 leads to a reduction in Robo2 mRNA and protein within SACMNs and SACMN axons fail to exit the spinal cord in Robo2-deficient mice. Consistent with short-range interactions between Robo2 and Slit ligands regulating SACMN axon exit, Robo2-expressing SACMN axons normally navigate through LEP-associated Slits as they emerge from the spinal cord, and fail to exit in Slit-deficient mice. Our studies support the view that Nkx2.9 controls SACMN axon exit from the mammalian spinal cord by regulating Robo-Slit signaling.

Forward signaling by EphB1/EphB2 interacting with ephrin-B ligands at the optic chiasm is required to form the ipsilateral projection

EphB receptor tyrosine kinases direct axonal pathfinding through interactions with ephrin-B proteins following axon-cell contact. As EphB:ephrin-B binding leads to bidirectional signals, the contributions of signaling into the Eph-expressing cell (forward signaling) or the ephrin-expressing cell (reverse signaling) cannot be assigned using traditional protein null alleles. To determine if EphB1 is functioning solely as a receptor during axon pathfinding, a new knock-in mutant mouse was created, EphB1(T-lacZ), which expresses an intracellular-truncated EphB1-β-gal fusion protein from the endogenous locus. As in the EphB1(-/-) protein null animals, the EphB1(T-lacZ/T-lacZ) homozygotes fail to form the ipsilateral projecting subpopulation of retinal ganglion cell axons. This indicates that reverse signaling through the extracellular domain of EphB1 is not required for proper axon pathfinding of retinal axons at the optic chiasm. Further analysis of other EphB and ephrin-B mutant mice shows that EphB1 is the preferred receptor of ephrin-B2 and, to a lesser degree, ephrin-B1 in mediating axon guidance at the optic chiasm despite the coexpression of EphB2 in the same ipsilaterally projecting retinal axons.

Ephrin reverse signaling in axon guidance and synaptogenesis

Axon-cell and axon-dendrite contact is a highly regulated process necessary for the formation of precise neural circuits and a functional neural network. Eph-ephrin interacting molecules on the membranes of axon nerve terminals and target dendrites act as bidirectional ligands/receptors to transduce signals into both the Eph-expressing and ephrin-expressing cells to regulate cytoskeletal dynamics. In particular, recent evidence indicates that ephrin reverse signal transduction events are important in controlling both axonal and dendritic elaborations of neurons in the developing nervous system. Here we review how ephrin reverse signals are transduced into neurons to control maturation of axonal pre-synaptic and dendritic post-synaptic structures.

Combining modularity, conservation, and interactions of proteins significantly increases precision and coverage of protein function prediction

BACKGROUND

While the number of newly sequenced genomes and genes is constantly increasing, elucidation of their function still is a laborious and time-consuming task. This has led to the development of a wide range of methods for predicting protein functions in silico. We report on a new method that predicts function based on a combination of information about protein interactions, orthology, and the conservation of protein networks in different species.

RESULTS

We show that aggregation of these independent sources of evidence leads to a drastic increase in number and quality of predictions when compared to baselines and other methods reported in the literature. For instance, our method generates more than 12,000 novel protein functions for human with an estimated precision of ~76%, among which are 7,500 new functional annotations for 1,973 human proteins that previously had zero or only one function annotated. We also verified our predictions on a set of genes that play an important role in colorectal cancer (MLH1, PMS2, EPHB4 ) and could confirm more than 73% of them based on evidence in the literature.

CONCLUSIONS

The combination of different methods into a single, comprehensive prediction method infers thousands of protein functions for every species included in the analysis at varying, yet always high levels of precision and very good coverage.

Activation of ephrin A proteins influences hematopoietic stem cell adhesion and trafficking patterns

OBJECTIVE

To determine if Eph receptors and ephrins can modulate the homing of hematopoietic cells in a murine bone marrow transplantation model.

MATERIALS AND METHODS

EphA and ephrin A gene expression by mouse hematopoietic stem cells and the progenitor cell line FDCP-1 was determined by real-time reverse transcription polymerase chain reaction and flow cytometry. The effect of ephrin A activation on adhesion of hematopoietic progenitors was determined by in vitro adhesion assays in which cells were exposed to fibronectin or vascular cell adhesion molecule-1 (VCAM-1) and an increasing gradient of immobilized EphA3-Fc. Adhesion to fibronectin and VCAM-1 was further investigated using soluble preclustered EphA3-Fc. We used soluble unclustered EphA3-Fc as an antagonist to block endogenous EphA-ephrin A interactions in vivo. The effect of injecting soluble EphA3-Fc on the mobilization of hematopoietic progenitor cells was examined. We determined the effect on short-term homing by pretreating bone marrow cells with EphA3-Fc or the control IgG before infusion into lethally irradiated mice.

RESULTS

Preclustered and immobilized EphA3-Fc increased adhesion of progenitor cells and FDCP-1 to fibronectin and VCAM-1 (1.6- to 2-fold higher adhesion; p < 0.05) relative to control (0 μ/cm(2) EphA3-Fc extracellular molecule alone). Injection of the antagonist soluble EphA3-Fc increased progenitor cell and colony-forming unit-spleen cells in the peripheral blood (42% greater colony-forming unit in culture; p < 0.05, 3.8-fold higher colony-forming unit-spleen) relative to control.

CONCLUSION

Treating bone marrow cells with EphA3-Fc resulted in a reduction by 31% in donor stem cells homing to the bone marrow and accumulation of donor cells in recipient spleens (50% greater than control) and greater recovery of donor stem cells from the peripheral blood.

Robo2 is required for Slit-mediated intraretinal axon guidance

The developing optic pathway has proven one of the most informative model systems for studying mechanisms of axon guidance. The first step in this process is the directed extension of retinal ganglion cell (RGC) axons within the optic fibre layer (OFL) of the retina towards their exit point from the eye, the optic disc. Previously, we have shown that the inhibitory guidance molecules, Slit1 and Slit2, regulate two distinct aspects of intraretinal axon guidance in a region-specific manner. Using knockout mice, we have found that both of these guidance activities are mediated via Robo2. Of the four vertebrate Robos, only Robo1 and Robo2 are expressed by RGCs. In mice lacking robo1 intraretinal axon guidance occurs normally. However, in mice lacking robo2 RGC axons make qualitatively and quantitatively identical intraretinal pathfinding errors to those reported previously in Slit mutants. This demonstrates clearly that, as in other regions of the optic pathway, Robo2 is the major receptor required for intraretinal axon guidance. Furthermore, the results suggest strongly that redundancy with other guidance signals rather than different receptor utilisation is the most likely explanation for the regional specificity of Slit function during intraretinal axon pathfinding.

Ephrin-B1 regulates axon guidance by reverse signaling through a PDZ-dependent mechanism

Mutations in the ephrin-B1 gene result in craniofrontonasal syndrome (CFNS) in humans, a congenital disorder that includes a wide range of craniofacial, skeletal, and neurological malformations. In addition to the ability of ephrin-B1 to forward signal through its cognate EphB tyrosine kinase receptors, ephrin-B1 can also act as a receptor and transduce a reverse signal by either PDZ-dependent or phosphorylation-dependent mechanisms. To investigate how ephrin-B1 acts to influence development and congenital disease, we generated mice harboring a series of targeted point mutations in the ephrin-B1 gene that independently ablate specific reverse signaling pathways, while maintaining forward signaling capacity. We demonstrate that both PDZ and phosphorylation-dependent reverse signaling by ephrin-B1 are dispensable for craniofacial and skeletal development, whereas PDZ-dependent reverse signaling by ephrin-B1 is critical for the formation of a major commissural axon tract, the corpus callosum. Ephrin-B1 is strongly expressed within axons of the corpus callosum, and reverse signaling acts autonomously in cortical axons to mediate an avoidance response to its signaling partner EphB2. These results demonstrate the importance of PDZ-dependent reverse signaling for a subset of Ephrin-B1 developmental roles in vivo.

Key roles of Ephs and ephrins in retinotectal topographic map formation

Cellular and molecular mechanisms involved in the development of topographic ordered connections in the central nervous system (CNS) constitute a key issue in neurobiology because neural connectivities are the base of the CNS normal function. We discuss the roles of the Eph/ephrin system in the establishment of retinotopic projections onto the tectum/colliculus, the most detailed studied model of topographic mapping. The expression patterns of Ephs and ephrins in opposing gradients both in the retina and the tectum/colliculus, label the local addresses on the target and give specific sensitivities to growth cones according to their topographic origin in the retina. We postulate that the highest levels of these gradients could signal both the entry as well as the limiting boundaries of the target. Since Ephs and ephrins are membrane-bound molecules, they may function as both receptors and ligands producing repulsive or attractant responses according to their microenvironment and play central roles in a variety of developmental events such as axon guidance, synapse formation and remodeling. Due to different experimental approaches and the inherent species-specific differences, some results appear contradictory and should be reanalyzed. Nevertheless, these studies about the roles of the Eph/ephrin system in retinotectal/collicular mapping support general principles in order to understand CNS development and could be useful to design regeneration therapies.

Conformal geometry of the retinal nerve fiber layer

The nerve fiber layer of the human retina is made up of the retinal segments of ganglion cell axons. Its geometry can be described mathematically as a fibration of a 2D domain: a partition of a certain region into smooth curves. Here, we present a simple family of curves that closely models the observed geometry of the nerve fiber layer. For each retina, the pattern depends on 2 parameters, A and B: A computer program determines A and B for a given retina and the theory matches the retina with a standard deviation of approximately 6-8 degrees . These particular curves turn out to be the curves that would be generated if the growing ganglion cell axon tip moved down a gradient toward a source of diffusible neuroattractant at the disk and away from a weaker macular diffusible repellant. Thus, this model provides morphological evidence that diffusible substances provide positional information to the embryonic ganglion cell axons in finding their way to the optic nerve head.

Intraretinal projection of retinal ganglion cell axons as a model system for studying axon navigation

The initial step of retinal ganglion cell (RGC) axon pathfinding involves directed growth of RGC axons toward the center of the retina, the optic disc, a process termed "intraretinal guidance". Due to the accessibility of the system, and with various embryological, molecular and genetic approaches, significant progress has been made in recent years toward understanding the mechanisms involved in the precise guidance of the RGC axons. As axons are extending from RGCs located throughout the retina, a multitude of factors expressed along with the differentiation wave are important for the guidance of the RGC axons. To ensure that the RGC axons are oriented correctly, restricted to the optic fiber layer (OFL) of the retina, and exit the eye properly, different sets of positive and negative factors cooperate in the process. Fasciculation mediated by a number of cell adhesion molecules (CAMs) and modulation of axonal response to guidance factors provide additional mechanisms to ensure proper guidance of the RGC axons. The intraretinal axon guidance thus serves as an excellent model system for studying how different signals are regulated, modulated and integrated for guiding a large number of axons in three-dimensional space.

In vivo use of a nanoknife for axon microsurgery

OBJECTIVE

Microfabricated devices with nanoscale features have been proposed as new microinstrumentation for cellular and subcellular surgical procedures, but their effectiveness in vivo has yet to be demonstrated. In this study, we examined the in vivo use of 10 to 100 microm-long nanoknives with cutting edges of 20 nm in radius of curvature during peripheral nerve surgery.

METHODS

Peripheral nerves from anesthetized mice were isolated on a rudimentary microplatform with stimulation microelectrodes, and the nanoknives were positioned by a standard micromanipulator. The surgical field was viewed through a research microscope system with brightfield and fluorescence capabilities.

RESULTS

Using this assembly, the nanoknife effectively made small, 50 to 100 microm-long incisions in nerve tissue in vivo. This microfabricated device was also robust enough to make repeated incisions to progressively pare down the nerve as documented visually and by the accompanying incremental diminution of evoked motor responses recorded from target muscle. Furthermore, this nanoknife also enabled the surgeon to perform procedures at an unprecedented small scale such as the cutting and isolation of a small segment from a single constituent axon in a peripheral nerve in vivo. Lastly, the nanoknife material (silicon nitride) did not elicit any acute neurotoxicity as evidenced by the robust growth of axons and neurons on this material in vitro.

CONCLUSION

Together, these demonstrations support the concept that microdevices deployed in a neurosurgical environment in vivo can enable novel procedures at an unprecedented small scale. These devices are potentially the vanguard of a new family of microscale instrumentation that can extend surgical procedures down to the cellular scale and beyond.

Encapsulated Petri dish system for single-cell drug delivery and long-term time lapse microscopy

We have developed a system that allows focal drug application for cell culture microscopy. Single-cell drug delivery is achieved through the insertion of a patch-clamp-like micropipette in a microenvironment-controlled chamber mounted on a standard 35-mm Petri dish. The system has precise control of temperature, CO(2) concentration, and humidity, while preventing contamination during experiments. The use of standard Petri dishes allows long-term experiments by alternating in situ microscopy with incubator growth. Modern biological long-term experiments such as the characterization of drug effects on cell movement, axonal guidance, mitosis, apoptosis, differentiation, or volume regulation can be performed. The chamber is compatible with any inverted microscope without significant modifications.

Eph receptor and ephrin signaling in developing and adult brain of the honeybee (Apis mellifera)

Roles for Eph receptor tyrosine kinase and ephrin signaling in vertebrate brain development are well established. Their involvement in the modulation of mammalian synaptic structure and physiology is also emerging. However, less is known of their effects on brain development and their function in adult invertebrate nervous systems. Here, we report on the characterization of Eph receptor and ephrin orthologs in the honeybee, Apis mellifera (Am), and their role in learning and memory. In situ hybridization for mRNA expression showed a uniform distribution of expression of both genes across the developing pupal and adult brain. However, in situ labeling with Fc fusion proteins indicated that the AmEphR and Amephrin proteins were differentially localized to cell body regions in the mushroom bodies and the developing neuropiles of the antennal and optic lobes. In adults, AmEphR protein was localized to regions of synaptic contacts in optic lobes, in the glomeruli of antennal lobes, and in the medial lobe of the mushroom body. The latter two regions are involved in olfactory learning and memory in the honeybee. Injections of EphR-Fc and ephrin-Fc proteins into the brains of adult bees, 1 h before olfactory conditioning of the proboscis extension reflex, significantly reduced memory 24 h later. Experimental amnesia in the group injected with ephrin-Fc was apparent 1 h post-training. Experimental amnesia was also induced by post-training injections with ephrin-Fc suggesting a role in recall. This is the first demonstration that Eph molecules function to regulate the formation of memory in insects.

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

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

Molecular regulation of visual system development: more than meets the eye

Vertebrate eye development has been an excellent model system to investigate basic concepts of developmental biology ranging from mechanisms of tissue induction to the complex patterning and bidimensional orientation of the highly specialized retina. Recent advances have shed light on the interplay between numerous transcriptional networks and growth factors that are involved in the specific stages of retinogenesis, optic nerve formation, and topographic mapping. In this review, we summarize this recent progress on the molecular mechanisms underlying the development of the eye, visual system, and embryonic tumors that arise in the optic system.

Retroviral misexpression of cVax disturbs retinal ganglion cell axon fasciculation and intraretinal pathfinding in vivo and guidance of nasal ganglion cell axons in vivo

The transcription factor cVax (Vax2) is expressed in the ventral neural retina and restricted expression is a prerequisite for at least three prominent aspects of retinal dorsal-ventral patterning: polarized expression of EphB/B-ephrin molecules, the retinotectal projection and the distribution of rod photoreceptors across the retina. In the chick retina, the fasciculation pattern of ganglion cell axons also differs between the dorsal and ventral eye. To investigate the molecular mechanisms involved, the nerve fiber layer was analyzed after retroviral misexpression of several factors known to regulate the positional specification of retinal ganglion cells. Forced cVax expression ventralized the fasciculation pattern and caused axon pathfinding errors near the optic disc. Ectopic expression of different ephrin molecules indicated that axon fasciculation is, at least in part, mediated by the EphB system. Finally, we report that retroviral misexpression of cVax increased the pool of EphA4 receptors phosphorylated on tyrosine residues and altered the guidance preference of nasal axons in vitro. These results identify novel functions for cVax in intraretinal axon fasciculation and pathfinding as well as suggest a mechanism to explain how restricted cVax expression may influence map formation along the dorso-ventral and antero-posterior axes of the optic tectum.

Distribution of EphB receptors and ephrin-B1 in the developing vertebrate spinal cord

Contact-dependent interactions between EphB receptors and ephrin-B ligands mediate a variety of cell-cell communication events in the developing and mature central nervous system (CNS). These predominantly repulsive interactions occur at the interface between what are considered to be mutually exclusive EphB and ephrin-B expression domains. We previously used receptor and ligand affinity probes to show that ephrin-B ligands are expressed in the floor plate and within a dorsal region of the embryonic mouse spinal cord, while EphB receptors are present on decussated segments of commissural axons that navigate between these ephrin-B domains. Here we present the generation and characterization of two new monoclonal antibodies, mAb EfB1-3, which recognizes EphB1, EphB2, and EphB3, and mAb efrnB1, which is specific for ephrin-B1. We use these reagents and polyclonal antibodies specific for EphB1, EphB2, EphB3, or ephrin-B1 to describe the spatiotemporal expression patterns of EphB receptors and ephrin-B1 in the vertebrate spinal cord. Consistent with affinity probe binding, we show that EphB1, EphB2, and EphB3 are each preferentially expressed on decussated segments of commissural axons in vivo and in vitro, and that ephrin-B1 is expressed in a dorsal domain of the spinal cord that includes the roof plate. In contrast to affinity probe binding profiles, we show here that EphB1, EphB2, and EphB3 are present on the ventral commissure, and that EphB1 and EphB3 are expressed on axons that compose the dorsal funiculus. In addition, we unexpectedly find that mesenchymal cells, which surround the spinal cord and dorsal root ganglion, express ephrin-B1.

Temporal regulation of EphA4 in astroglia during murine retinal and optic nerve development

Eph receptors and their ephrin ligands play important roles in many aspects of visual system development. In this study, we characterized the spatial and temporal expression pattern of EphA4 in astrocyte precursor cell (APC) and astrocyte populations in the murine retina and optic nerve. EphA4 is expressed by immotile optic disc astrocyte precursor cells (ODAPS), but EphA4 is downregulated as these cells migrate into the retina. Surprisingly, mature astrocytes in the adult retina re-express EphA4. Within the optic nerve, EphA4 is expressed in specialized astrocytes that form a meshwork at the optic nerve head (ONH). Our in vitro and in vivo data indicate that EphA4 is dispensable for retinal ganglion cell (RGC) axon growth and projections through the chiasm. While optic stalk structure, APC proliferation and migration, retinal vascularization, and oligodendrocyte migration appear normal in EphA4 mutants, the expression of EphA4 in APCs and in the astrocyte meshwork at the ONH has implications for optic nerve pathologies.

Pretarget sorting of retinocollicular axons in the mouse

The map of the retina onto the optic tectum is a highly conserved feature of the vertebrate visual system; the mechanism by which this mapping is accomplished during development is a long-standing problem of neurobiology. The early suggestion by Roger Sperry that the map is formed through interactions between retinal ganglion cell axons and target cells within the tectum has gained significant experimental support and widespread acceptance. Nonetheless, reports in a variety of species indicate that some aspects of retinotopic order exist within the optic tract, leading to the suggestion that this "preordering" of retinal axons may play a role in the formation of the mature tectal map. A satisfactory account of pretarget order must provide the mechanism by which such axon order develops. Insofar as this mechanism must ultimately be determined genetically, the mouse suggests itself as the natural species in which to pursue these studies. Quantitative and repeatable methods are required to assess the contribution of candidate genes in mouse models. For these reasons, we have undertaken a quantitative study of the degree of retinotopic order within the optic tract and nerve of wild-type mice both before and after the development of the retinotectal map. Our methods are based on tract tracing using lipophilic dyes, and our results indicate that there is a reestablishment of dorsoventral but not nasotemporal retinal order when the axons pass through the chiasm and that this order is maintained throughout the subsequent tract. Furthermore, this dorsoventral retinotopic order is well established by the day after birth, long before the final target zone is discernible within the tectum. We conclude that pretarget sorting of axons according to origin along the dorsoventral axis of the retina is both spatially and chronologically appropriate to contribute to the formation of the retinotectal map, and we suggest that these methods be used to search for the molecular basis of such order by using available mouse genetic models.

GRIP1 controls dendrite morphogenesis by regulating EphB receptor trafficking

The function of the multi-PDZ domain scaffold protein GRIP1 (glutamate receptor interacting protein 1) in neurons is unclear. To explore the function of GRIP1 in hippocampal neurons, we used RNA interference (RNAi) to knock down the expression of GRIP1. Knockdown of GRIP1 by small interfering RNA (siRNA) in cultured hippocampal neurons caused a loss of dendrites, associated with mislocalization of the GRIP-interacting proteins GIuR2 (AMPA receptor subunit), EphB2 (receptor tyrosine kinase) and KIF5 (also known as kinesin 1; microtubule motor). The loss of dendrites by GRIP1-siRNA was rescued by overexpression of the extracellular domain of EphB2, and was phenocopied by overexpression of the intracellular domain of EphB2 and extracellular application of ephrinB-Fc fusion proteins. Neurons from EphB1-EphB2-EphB3 triple knockout mice showed abnormal dendrite morphogenesis. Disruption of the KIF5-GRIP1 interaction inhibited EphB2 trafficking and strongly impaired dendritic growth. These results indicate an important role for GRIP1 in dendrite morphogenesis by serving as an adaptor protein for kinesin-dependent transport of EphB receptors to dendrites.

Optic nerve axons: life and death before birth

In the 2004 Bowman Lecture, I give a panegyric for Sir William Bowman, an estimate of the importance and the epidemiology of anterior visual pathway developmental disorders, followed by a history of the anterior visual system. I review the normal development of the optic nerve and chiasm and the main developmental disorders: Optic Nerve Aplasia, Optic Nerve Hypoplasia and Achiasmia.

High EphA3 expressing ophthalmic trigeminal sensory axons are sensitive to ephrin-A5-Fc: Implications for lobe specific axon guidance

The ophthalmic, maxillary and mandibular axon branches of the trigeminal ganglion provide cutaneous sensory innervation to the vertebrate face. In the chick embryo, the trigeminal ganglion is bilobed, with ophthalmic axons projecting from the ophthalmic lobe, while maxillary and mandibular projections emerge from the maxillomandibular lobe. To date, target tissue specific guidance cues that discriminately guide the axon projections from the two trigeminal ganglion lobes are unknown. EphA receptor tyrosine kinases and ephrin-A ligands are excellent candidates for this process as they are known to mediate axon guidance in the developing nervous system. Accordingly, the expression of EphAs and ephrin-As was investigated at stages 13, 15, 20 of chick embryogenesis when peripheral axons from the trigeminal ganglion are pathfinding. EphA3 is expressed highly in the ophthalmic trigeminal ganglion lobe neurons in comparison to maxillomandibular trigeminal ganglion lobe neurons. Furthermore, from stages 13-20 ephrin-A2 and ephrin-A5 ligands are only localized to the mesenchyme of the first branchial arch (maxillary and mandibular processes), the target fields for maxillomandibular trigeminal ganglion axons. We found that ophthalmic and not maxillomandibular lobe axons were responsive to ephrin-A5-Fc utilizing a substratum choice assay. The implication of these results is that EphA3 forward signaling in ophthalmic sensory axons may be an important mechanism in vivo for lobe specific guidance of trigeminal ganglion ophthalmic projections.

Molecular definition of an in vitro niche for dendritic cell development

OBJECTIVE

Although dendritic cell (DC) precursors have been isolated from many lymphoid sites, the regulation and location of early DC development is still poorly understood. Here we describe a splenic microenvironment that supports DC hematopoiesis in vitro and identify gene expression specific for that niche.

METHODS

The DC supportive function of the STX3 splenic stroma and the lymph node-derived 2RL22 stroma for overlaid bone marrow cells was assessed by coculture over 2 weeks. The DC supportive function of SXT3 was identified in terms of specific gene expression in STX3 and not 2RL22 using Affymetrix microchips.

RESULTS

STX3 supports DC differentiation from overlaid bone marrow precursors while 2RL22 does not. A dataset of 154 genes specifically expressed in STX3 and not 2RL22 was retrieved from Affymetrix results. Functional annotation has led to selection of 26 genes as candidate regulators of the microenvironment supporting DC hematopoiesis. Specific expression of 14 of these genes in STX3 and not 2RL22 was confirmed by reverse transcription-polymerase chain reaction.

CONCLUSION

Some genes specifically expressed in STX3 have been previously associated with hematopoietic stem cell niches. A high proportion of genes encode growth factors distinct from those commonly used for in vitro development of DC from precursors. Potential regulators of a DC microenvironment include genes involved in angiogenesis, hematopoiesis, and development, not previously linked to DC hematopoiesis.

Ganglion cell axon pathfinding in the retina and optic nerve

The eye is a highly specialized structure that gathers and converts light information into neuronal signals. These signals are relayed along axons of retinal ganglion cells (RGCs) to visual centers in the brain for processing. In this review, we discuss the pathfinding tasks RGC axons face during development and the molecular mechanisms known to be involved. The data at hand support the presence of multiple axon guidance mechanisms concentrically organized around the optic nerve head, each of which appears to involve both growth-promoting and growth-inhibitory guidance molecules. Together, these strategies ensure proper optic nerve formation and establish the anatomical pathway for faithful transmission of information between the retina and the brain.

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

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

Neurite elongation on chondroitin sulfate proteoglycans is characterized by axonal fasciculation

In the developing or regenerating nervous system, migrating growth cones are exposed to regulatory molecules that positively and/or negatively affect guidance. Chondroitin sulfate proteoglycans (CSPGs) are complex macromolecules that are typically negative regulators of growth cone migration in vivo and in vitro. However, in certain cases, neurites sometimes traverse regions expressing relatively high levels of CSPGs, seemingly a paradox. In our continuing efforts to characterize CSPG inhibition in vitro, we manipulated the ratio of CSPGs to growth-promoting laminin-1 to produce a substratum that supports outgrowth of a subpopulation of dorsal root ganglia (DRG) neurites, while still being inhibitory to other populations of DRG neurons [Exp. Neurol. 109 (1990), 111; J. Neurobiol. 51 (2002), 285]. This model comprises a useful tool in the analysis of mechanisms of growth cone guidance and is particularly useful to analyze how CSPGs can be inhibitory under some conditions, and growth permissive under others. We grew embryonic (E9-10) chicken DRG neurons on nervous system-isolated, substratum-bound CSPGs at a concentration that supports an intermittent pattern of outgrowth, alternating with regions adsorbed with growth-promoting laminin-1 alone, and analyzed outgrowth behaviors qualitatively and quantitatively. A novel finding of the study was that DRG neurites that elongated onto CSPGs were predominantly fasciculated, but immediately returned to a defasciculated state upon contact with laminin-1. Further, cursory inspection suggests that outgrowth onto CSPGs may be initially accomplished by pioneer axons, along which subsequent axons migrate. The outgrowth patterns characterized in vitro may accurately reflect outgrowth in vivo in locations where inhibitory CSPGs and growth-promoting molecules are coexpressed, e.g., in the developing retina where fasciculated outgrowth may be instrumental in the guidance of retinal ganglion cells from the periphery to the optic fissure.

EphA4 provides repulsive signals to developing cochlear ganglion neurites mediated through ephrin-B2 and -B3

The ephrins and Eph receptors make up two large families of bi-directional signaling molecules that are known to play a role in the development of the nervous system. Recently, expression of EphA4 in the developing cochlea was shown, with strong expression in cells lining the osseous spiral lamina (OSL) through which afferent dendrites must pass to reach the organ of Corti (OC). It was also demonstrated that ephrin-B2 and -B3, both of which are known to interact with EphA4, are expressed by spiral ganglion (SG) neurons. To investigate the functional role of EphA4 in the development of inner ear neurons, neonatal rat SG explants were cultured for 72 hours on uniformly coated surfaces or near stripes of EphA4/IgG-Fc-chimera. Control explants were cultured on or near IgG-Fc and EphA1/IgG-Fc-chimera. To assess the roles of ephrin-B2 and -B3 in EphA4 signaling, SG explants were cultured with or without anti-ephrin-B2 and/or -B3 blocking antibodies. Growth patterns of SG neurites at the border of EphA4 receptor stripes showed repulsion, characterized by turning, stopping and/or reversal. In the case of IgG-Fc and EphA1, the neurites grew straight onto the stripes. Treatment with either anti-ephrin-B2 or -B3 blocking antibodies significantly reduced the repulsive effect of an EphA4 stripe. Moreover, when both antibodies were used together, neurites crossed onto EphA4 stripes with no evidence of repulsion. The results suggest that EphA4 provides repulsive signals to SG neurites in the developing cochlea, and that ephrin-B2 and -B3 together mediate this response.

Ephrin-B1 reverse signaling activates JNK through a novel mechanism that is independent of tyrosine phosphorylation

Eph receptors and their cognate ligand ephrins play important roles in various biological processes such as cell migration, axon guidance, and synaptic plasticity. One characteristic feature of the Eph-ephrin signal transduction is that, upon interaction with the receptor, the transmembrane B-class ephrins become tyrosine-phosphorylated and transduce intracellular signals that lead to reorganization of the cytoskeleton. Although in vitro and genetic studies have demonstrated unequivocally the significance of this reverse signaling, the underlying mechanism remains unclear. We report here that transfection of ephrin-B1 into 293 cells resulted in robust increase in JNK activity, whereas expression of truncated ephrin-B1 lacking the cytoplasmic domain had a negligible effect, indicating that the induction of JNK activity was attributed mainly to the reverse signaling. The ephrin-B1-mediated JNK activation was reduced significantly by dominant-negative TAK1, MKK4, or MKK7. Ephrin-B1 over-expressing 293 cells became rounded in morphology. Surprisingly, ephrin-B1 that lacked all six intracellular tyrosine residues still triggered JNK activation and rounding morphology of the transfected cells. Consistent with these observations, activation of JNK and the resulting morphological changes mediated by ephrin-B1 could be abolished by the JNK inhibitor SP600125 but not the Src inhibitor PP2. Taken together, our findings have identified a novel reverse signaling pathway transduced by ephrin-B1, which is independent of tyrosine phosphorylation but involves the activation of JNK through TAK1 and MKK4/MKK7 and leads to changes in cell morphology.

EphB receptors influence growth of ephrin-B1-positive statoacoustic nerve fibers

The Eph family of receptors and ligands has been implicated in a variety of developmental processes, including the provision of inhibitory guidance cues to developing nerve fibers. A unique property of the B class of receptors is that they are able to phosphorylate ephrin-B ligands, allowing for bi-directional, or reverse signalling. While most of the studies to date have focused on central nerve fibers, little is known about the role of Eph molecules in guiding nerve fibers of the peripheral nervous system. In the present study, ephrin-B1 was found to be highly expressed on developing peripheral nerve fibers including auditory and vestibular (statoacoustic) and dorsal root ganglion nerve fibers. In vitro assays revealed that EphB-Fc receptors inhibited further growth of statoacoustic nerve fibers. In contrast, EphA7-Fc and ephrin-B2-Fc did not prevent further growth of SAG. Together, these results suggest a role for EphB receptors in providing guidance signals to ephrin-B1-positive SAG nerve fibers.

Understanding dorsoventral topography: backwards and forwards

Retinal axons project to their central targets along two orthogonal topographic axes, anterior-posterior (A-P) and dorsal-ventral (D-V). While ephrin-A/EphA signaling determines A-P topography, little has been known about the molecular mechanisms guiding axons along the D-V axis. Two papers by Mann et al. and Hindges et al. in this issue of Neuron provide evidence for both forward and reverse ephrin-B/EphB signaling in regulating D-V topography.

Topographic mapping in dorsoventral axis of the Xenopus retinotectal system depends on signaling through ephrin-B ligands

Ephrin-B and EphB are distributed in matching dorsoventral gradients in the embryonic Xenopus visual system with retinal axons bearing high levels of ligand (dorsal) projecting to tectal regions with high receptor expression (ventral). In vitro stripe assays show that dorsal retinal axons prefer to grow on EphB receptor stripes supporting an attractive guidance mechanism. In vivo disruption of EphB/ephrin-B function by application of exogenous EphB or expression of dominant-negative ephrin-B ligand in dorsal retinal axons causes these axons to shift dorsally in the tectum, while misexpression of wild-type ephrin-B in ventral axons causes them to shift ventrally. These dorsoventral targeting errors are consistent with the hypothesis that an attractive mechanism that requires ephrin-B cytoplasmic domain is critical for retinotectal mapping in this axis.

Ephrins in reverse, park and drive

Eph receptors and their membrane-anchored ephrin ligands are thought to orchestrate cell movements by transducing bidirectional tyrosine-kinase-mediated signals into both cells expressing the receptors and cells expressing the ligands. Whether the resulting event is repulsion of an axonal growth cone, directing the orderly segmentation of hindbrain rhombomere cells or controlling angiogenic remodelling, such elaborate and diverse cell movements require intricate changes in the actin cytoskeleton, as well as precise regulation of cellular adhesion. Recent work by several groups has begun to link ephrin reverse signals to intracellular pathways that regulate actin dynamics and might help to explain how these ligands function as receptors to direct cell movement, adhesion and de-adhesion events.

Axon guidance: the cytoplasmic tail

Recent advances in the study of axon guidance have begun to clarify the intricate signalling mechanisms utilised by receptors that mediate path-finding. Many of these axon guidance receptors, including Plexin B, EphA, ephrin B and Robo, regulate the Rho family of GTPases, to effect changes in motility. Recent studies demonstrate a critical role for the cytoplasmic tails of guidance receptors in signalling and also reveal the potential for a great deal of crosstalk between the various receptor-signalling pathways.