Engrailed and retinotectal topography

H2O2 and Engrailed 2 paracrine activity synergize to shape the zebrafish optic tectum

Although a physiological role for redox signaling is now clearly established, the processes sensitive to redox signaling remains to be identified. Ratiometric probes selective for H₂O₂ have revealed its complex spatiotemporal dynamics during neural development and adult regeneration and perturbations of H₂O₂ levels disturb cell plasticity and morphogenesis. Here we ask whether endogenous H₂O₂ could participate in the patterning of the embryo. We find that perturbations of endogenous H₂O₂ levels impact on the distribution of the Engrailed homeoprotein, a strong determinant of midbrain patterning. Engrailed 2 is secreted from cells with high H₂O₂ levels and taken up by cells with low H₂O₂ levels where it leads to increased H₂O₂ production, steering the directional spread of the Engrailed gradient. These results illustrate the interplay between protein signaling pathways and metabolic processes during morphogenetic events.

The Physiology of Homeoprotein Transduction

The homeoprotein family comprises ~300 transcription factors and was long seen as primarily involved in developmental programs through cell autonomous regulation. However, recent evidence reveals that many of these factors are also expressed in the adult where they exert physiological functions not yet fully deciphered. Furthermore, the DNA-binding domain of most homeoproteins contains two signal sequences allowing their secretion and internalization, thus intercellular transfer. This review focuses on this new-found signaling in cell migration, axon guidance, and cerebral cortex physiological homeostasis and speculates on how it may play important roles in early arealization of the neuroepithelium. It also describes the use of homeoproteins as therapeutic proteins in mouse models of diseases affecting the central nervous system, in particular Parkinson disease and glaucoma.

En1 is necessary for survival of neurons in the ventral nuclei of the lateral lemniscus

The ventral nuclei of the lateral lemniscus (VNLL) are part of the central auditory system thought to participate in temporal sound processing. While the timing and location of VNLL neurogenesis have been determined, the genetic factors that regulate VNLL neuron development are unknown. Here, we use genetic fate-mapping techniques to demonstrate that all glycinergic and glycinergic/GABAergic VNLL neurons derive from a cellular lineage that expresses the homeobox transcription factor Engrailed 1 (En1). We also show that En1 deletion does not affect migration or adoption of a neuronal cell fate but does lead to VNLL neuron death during development. Furthermore, En1 deletion blocks expression of the transcription factor FoxP1 in a subset of VNLL neurons. Together, these data identify En1 as a gene important for VNLL neuron development and survival. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1266-1274, 2016.

Engrailed homeoproteins in visual system development

Engrailed is a homeoprotein transcription factor. This family of transcription factors is characterized by their DNA-binding homeodomain and some members, including Engrailed, can transfer between cells and regulate protein translation in addition to gene transcription. Engrailed is intimately involved in the development of the vertebrate visual system. Early expression of Engrailed in dorsal mesencephalon contributes to the development and organization of a visual structure, the optic tectum/superior colliculus. This structure is an important target for retinal ganglion cell axons that carry visual information from the retina. Engrailed regulates the expression of Ephrin axon guidance cues in the tectum/superior colliculus. More recently it has been reported that Engrailed itself acts as an axon guidance cue in synergy with the Ephrin system and is proposed to enhance retinal topographic precision.

Engrailed signaling in axon guidance and neuron survival

Several homeoproteins can function in a direct cell non-autonomous fashion to control various biological processes. In the developing nervous system, this mode of signaling has been well documented for Engrailed in the guidance of retinal ganglion cell axons and retino-tectal patterning. Engrailed is also a key factor for mesencephalic dopaminergic (mDA) neurons, not only during development but also in the adult. Haplodeficiency for Engrailed1 leads to progressive adult-onset loss of mDA neurons and several phenotypic alterations reminiscent of Parkinson's disease (PD). Thanks to its transduction properties, Engrailed has been shown to confer neuroprotection in several experimental models of PD. Study of the mechanisms underlying these two Engrailed-mediated effects has revealed a key role of the translation regulation by Engrailed and uncovered an unsuspected link between a homeoprotein and mitochondrial activity. These studies highlight the crucial role of cellular energetic metabolism in neuron development, survival and neurodegeneration, and may help to identify novel therapeutic targets.

Hypoxanthine-guanine phosphoribosyl transferase regulates early developmental programming of dopamine neurons: implications for Lesch-Nyhan disease pathogenesis

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency results in Lesch-Nyhan disease (LND), where affected individuals exhibit a characteristic neurobehavioral disorder that has been linked with dysfunction of dopaminergic pathways of the basal ganglia. Since the functions of HPRT, a housekeeping enzyme responsible for recycling purines, have no direct relationships with the dopaminergic pathways, the mechanisms whereby HPRT deficiency affect them remain unknown. The current studies demonstrate that HPRT deficiency influences early developmental processes controlling the dopaminergic phenotype, using several different cell models for HPRT deficiency. Microarray methods and quantitative PCR were applied to 10 different HPRT-deficient (HPRT(-)) sublines derived from the MN9D cell line. Despite the variation inherent in such mutant sublines, several consistent abnormalities were evident. Most notable were increases in the mRNAs for engrailed 1 and 2, transcription factors known to play a key role in the specification and survival of dopamine neurons. The increases in mRNAs were accompanied by increases in engrailed proteins, and restoration of HPRT reverted engrailed expression towards normal levels, demonstrating a functional relationship between HPRT and engrailed. The functional relevance of the abnormal developmental molecular signature of the HPRT(-) MN9D cells was evident in impoverished neurite outgrowth when the cells were forced to differentiate chemically. To verify that these abnormalities were not idiosyncratic to the MN9D line, HPRT(-) sublines from the SK-N-BE(2) M17 human neuroblastoma line were evaluated and an increased expression of engrailed mRNAs was also seen. Over-expression of engrailed occurred even in primary fibroblasts from patients with LND in a manner that suggested a correlation with disease severity. These results provide novel evidence that HPRT deficiency may affect dopaminergic neurons by influencing early developmental mechanisms.

Developmental mechanisms for retinal degeneration in the blind cavefish Astyanax mexicanus

The sighted surface-dwelling (surface fish, SF) and the blind cave-living (cavefish, CF) forms of Astyanax mexicanus offer a unique opportunity to study the evolutionary changes in developmental mechanisms that lead to retinal degeneration. Previous data have shown the role of increased midline Sonic Hedgehog (Shh) signalling in cavefish eye degeneration (Yamamoto et al. [2004] Nature 431:844-847). Here, we have compared the major steps of eye development in SF and CF between 14 hours and 5 days of development. We have analyzed cell proliferation through PCNA and phospho-histone H3 staining and apoptosis through TUNEL and live LysoTracker analysis. We have assessed the expression of the major eye development signalling factors Shh and Fgf8, and the eye patterning genes Pax6, Lhx2, Lhx9, and Vax1, together with the differentiation marker GAD65. We show that eye development is retarded in CF and that cell proliferation in CF retina is proportionately similar to SF during early development, yet the retina degenerates after massive apoptosis in the lens and widespread cell death throughout the neuroretina. Moreover, and surprisingly, the signalling, patterning, and differentiation processes leading to the establishment of retinal layers and cell types happen almost normally in CF, although some signs of disorganization, slight heterochronies, and a lack of expression gradients are observable. Our data demonstrate that the evolutionary process of eye degeneration in the blind CF does not occur because of patterning defects of the retina and are consistent with the proposed scenario in which the trigger for eye degeneration in CF is lens apoptosis.

Development of continuous and discrete neural maps

Two qualitatively different kinds of neural map have been described: continuous maps exemplified by the visual retinotopic map, and discrete maps exemplified by the olfactory glomerular map. Here, we review developmental mechanisms of retinotopic and olfactory glomerular mapping and discuss underlying commonalities that have emerged from recent studies. These include the use of molecular gradients, axon-axon interactions, and the interplay between labeling molecules and neuronal activity in establishing these maps. Since visual retinotopic and olfactory glomerular maps represent two ends of a continuum that includes many other types of neural map in between, these emerging general principles may be widely applicable to map formation throughout the nervous system.

The topological role of homeoproteins in the developing central nervous system

Homeogenes encode homeoprotein transcription factors that have fundamental roles in development. They are key players in genetic networks that lay out the body plan and also determine morphology and physiology at the cellular and multicellular level. However, homeoproteins share activities that extend beyond transcription, including translation regulation and signalling. For example, homeoproteins participate in the definition of territories in the neuroepithelium and also have a function in axonal guidance. Based on these examples, we propose that homeoproteins are not only morphogenetic transcription factors, but also morphogens themselves.

Direct non-cell autonomous Pax6 activity regulates eye development in the zebrafish

Background

Modifications in Pax6 homeogene expression produce strong eye phenotypes. This suggested to us that eye development might be an appropriate model to verify if homeoprotein intercellular passage has important functions in early development. Similar to other homeoproteins, Pax6 has two domains that enable secretion and internalization by live cells and, thus, intercellular passage. In principle, a straightforward way to test the hypothesis would be to mutate one of the two sequences to produce a 'cell autonomous only' Pax6. However, this was not possible because these sequences are in the homeodomain and their modification would affect Pax6 transcriptional properties. We have thus developed an approach aimed at blocking Pax6 only in the extracellular milieu of developing zebrafish embryos.

Results

A first strategy was to inject a one-cell embryo with a mRNA encoding a secreted single-chain anti-Pax6 antibody. A second, complementary, strategy was to inject a Pax6 antibody in the blastula extracellular milieu. In both cases, 'dissymmetric eyes', 'one eye only' and 'no eye' phenotypes were produced. In most cases, lens phenotypes paralleled retina malformations. Although eye phenotypes were analyzed 30 hours post-fertilization, there was a strong correlation between early eye field asymmetry, early asymmetry in Pax6 expression and later-occurring eye malformations. Several controls were introduced, demonstrating that the effect is specific to Pax6 and cannot be explained by intracellular antibody activities.

Conclusion

This study supports the hypothesis that the Pax6 transcription factor is also a signaling molecule with direct non-cell autonomous activity.

The transcription factor Engrailed-2 guides retinal axons

Engrailed-2 (En-2), a homeodomain transcription factor, is expressed in a caudal-to-rostral gradient in the developing midbrain, where it has an instructive role in patterning the optic tectum--the target of topographic retinal input. In addition to its well-known role in regulating gene expression through its DNA-binding domain, En-2 may also have a role in cell-cell communication, as suggested by the presence of other domains involved in nuclear export, secretion and internalization. Consistent with this possibility, here we report that an external gradient of En-2 protein strongly repels growth cones of Xenopus axons originating from the temporal retina and, conversely, attracts nasal axons. Fluorescently tagged En-2 accumulates inside growth cones within minutes of exposure, and a mutant form of the protein that cannot enter cells fails to elicit axon turning. Once internalized, En-2 stimulates the rapid phosphorylation of proteins involved in translation initiation and triggers the local synthesis of new proteins. Furthermore, the turning responses of both nasal and temporal growth cones in the presence of En-2 are blocked by inhibitors of protein synthesis. The differential guidance of nasal and temporal axons reported here suggests that En-2 may participate directly in topographic map formation in the vertebrate visual system.

A single cell analysis of engrailed expression in the early embryonic brain of the grasshopper Schistocerca gregaria: ontogeny and identity of the secondary headspot cells

The expression pattern of the segment polarity gene engrailed was studied at the single cell level in the protocerebrum of the early embryonic brain of the grasshopper Schistocerca gregaria, the neuromere containing the secondary headspot cells. The engrailed protein is first expressed in the protocerebrum at about 22% of embryogenesis by a group of identified neuroblasts bordering the antennal lobe. The number of immunoreactive neuroblasts increases up to 26% of embryogenesis and then rapidly declines so that by 30% only the three most posterior remain immunoreactive. These three neuroblasts become incorporated into the developing antennal lobe of the deutocerebrum. Subsequently, there is a progressive re-expression of the engrailed protein in the protocerebrum by the so-called six secondary headspot cells. These are the first born sibling progeny of three identified protocerebral neuroblasts which themselves expressed the engrailed protein prior to generating their lineages, and so represents a reacquisition of engrailed expression within identified clones. The secondary headspot cells are neurons which direct axonal processes into the developing optic tract and so contribute to the primary axon scaffold of the brain. From our analysis of their ontogeny, we conclude that the secondary headspot cells do not represent a segmental border in the brain.

Dynamic regulation of axon guidance

To reach their proper targets, axons rely upon the actions of highly conserved families of attractive and repulsive guidance molecules, including the netrins, Slits, semaphorins and ephrins. These guidance systems are used to generate an astonishingly varied set of neuronal circuits. Here we consider the mechanisms by which a few guidance systems can be used to generate diverse outcomes. Recent studies have revealed extensive transcriptional and post-transcriptional regulation of guidance cues and their receptors, as well as combinatorial mechanisms that integrate information from different families of guidance cues.

Development of the visual system of the chick. II. Mechanisms of axonal guidance

The quest to understand axonal guidance mechanisms requires exact and multidisciplinary analyses of axon navigation. This review is the second part of an attempt to synthesise experimental data with theoretical models of the development of the topographic connection of the chick retina with the tectum. The first part included classic ideas from developmental biology and recent achievements on the molecular level in understanding cytodifferentiation and histogenesis [J. Mey, S. Thanos, Development of the visual system of the chick. (I) Cell differentiation and histogenesis, Brain Res. Rev. 32 (2000) 343-379]. The present part deals with the question of how millions of fibres exit from the eye, traverse over several millimetres and spread over the optic tectum to assemble a topographic map, whose precision accounts for the sensory performance of the visual system. The following topics gained special attention in this review. (i) A remarkable conceptual continuity between classic embryology and recent molecular biology has revealed that positional cellular specification precedes and determines the formation of the retinotectal map. (ii) Graded expression of asymmetric genes, transcriptional factors and receptors for signal transduction during early development seem to play a crucial role in determining the spatial identity of neurons within surface areas of retina and optic tectum. (iii) The chemoaffinity hypothesis constitutes the conceptual framework for development of the retinotopic organisation of the primary visual pathway. Studies of repulsive factors in vitro developed the original hypothesis from a theoretical postulate of chemoattraction to an empirically supported concept based on chemorepulsion. (iv) The independent but synchronous development of retina and optic tectum in topo-chronologically corresponding patterns ensures that ingrowing retinal axons encounter receptive target tissue at appropriate locations, and at the time when connections are due to be formed. (v) The growth cones of the retino-fugal axons seem to be guided both by local cues on glial endfeet and within the extracellular matrix. On the molecular level, the ephrins and their receptors have emerged as the most likely candidates for the material substrate of a topographic projection along the anterior-posterior axis of the optic tectum. Yet, since a number of alternative molecules have been proposed for the same function, it remains the challenge for the near future to define the proportional contribution of each one of the individual mechanisms proposed by matching theoretical predictions with the experimental evidence.

An approach to the complexity of the brain

The establishment of ordered neuronal connections is supposed to take place under the control of specific cell adhesion molecules (CAM) which guide neuroblasts and axons to their appropriate destination. The extreme complexity of the nervous system does not provide a favorable medium for the development of deterministic connections. Simon's [112] theorems offer a mean to approach the high level of complexity of the nervous system. The basic tenet is that complex systems are hierarchically organized and decomposable. Such systems can arise by selective trial and error mechanisms. Subsystems in complex systems only interact in an aggregate manner, and no significant information is lost if the detail of aggregate interactions is ignored. A number of nervous activities, which qualify for these requirements, are shown. The following sources of selection are considered: internal and external feedbacks, previous experience, plasticity in simple structures, and the characteristic geometry of dendrites. The role played by CAMs and other membrane-associated molecules is discussed in the sense that they are either inductor molecules that turn on different homeobox genes, or downstream products of genes, or both. These molecules control cellular and tissular differentiation in the developing brain creating sources of selection required for the trial and error process in the organization of the nervous tissue.

Two homeobox genes define the domain of EphA3 expression in the developing chick retina

Graded expression of the Eph receptor EphA3 in the retina and its two ligands, ephrin A2 and ephrin A5 in the optic tectum, the primary target of retinal axons, have been implicated in the formation of the retinotectal projection map. Two homeobox containing genes, SOHo1 and GH6, are expressed in a nasal-high, temporal-low pattern during early retinal development, and thus in opposing gradients to EphA3. Retroviral misexpression of SOHo1 or GH6 completely and specifically repressed EphA3 expression in the neural retina, but not in other parts of the central nervous system, such as the optic tectum. Under these conditions, some temporal ganglion cell axons overshot their expected termination zones in the rostral optic tectum, terminating aberrantly at more posterior locations. However, the majority of ganglion cell axons mapped to the appropriate rostrocaudal locations, although they formed somewhat more diffuse termination zones. These findings indicate that other mechanisms, in addition to differential EphA3 expression in the neural retina, are required for retinal ganglion axons to map to the appropriate rostrocaudal locations in the optic tectum. They further suggest that the control of topographic specificity along the retinal nasal-temporal axis is split into several independent pathways already at a very early time in development.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

The mammalian Odz gene family: homologs of a Drosophila pair-rule gene with expression implying distinct yet overlapping developmental roles

The Drosophila pair-rule gene odz (Tenm) has many patterning roles throughout development. We have identified four mammalian homologs of this gene, including one previously described as a mouse ER stress response gene, Doc4 (Wang et al., 1998). The Odz genes encode large polypeptides displaying the hallmarks of Drosophila Odz: a putative signal peptide; eight EGF-like repeats; and a putative transmembrane domain followed by a 1800-amino-acid stretch without homology to any proteins outside of this family. The mouse genes Odz3 and Doc4/Odz4 exhibit partially overlapping, but clearly distinct, embryonic expression patterns. The major embryonic sites of expression are in the nervous system, including the tectum, optic recess, optic stalk, and developing eye. Additional sites of expression include trachea and mesodermally derived tissues, such as mesentery, and forming limb and bone. Expression of the Odz2 gene is restricted to the nervous system. The expression patterns suggest that each of the genes has its own distinct developmental role. Comparisons of Drosophila and vertebrate Odz expression patterns suggest evolutionarily conserved functions.

Retinotectal maps: molecules, models and misplaced data

The mechanisms underlying the formation of topographic maps in the retinotectal system have long been debated. Recently, members of the Eph and ephrin receptor-ligand family have been found to provide a molecular substrate for one type of mechanism, that of chemospecific gradient matching, as proposed by Sperry. However, experiments over several decades have demonstrated that there is more to map formation than gradient matching. This article briefly reviews the old and new findings, argues that these two types of data must be properly integrated in order to understand map formation fully, and suggests some experimental and theoretical ways to begin this process.

Spatial correspondence between R-cadherin expression domains and retinal ganglion cell axons in developing zebrafish

Mechanisms underlying axonal pathfinding have been investigated for decades, and numerous molecules have been shown to play roles in this process, including members of the cadherin family of cell adhesion molecules. We showed in the companion paper that a member of the cadherin family (zebrafish R-cadherin) is expressed in retinal ganglion cells, and in presumptive visual structures in zebrafish brain, during periods when the axons were actively extending toward their targets. The present study extends the earlier work by using 1,1'-dioctadecyl-3,3,3',3', tetramethylindocarbocyanine perchlorate (DiI) anterograde tracing techniques to label retinal ganglion cell axons combined with R-cadherin in situ hybridization to explicitly examine the association ofretinal axons and brain regions expressing R-cadherin message. We found that in zebrafish embryos at 46-54 hours postfertilization, DiI-labeled retinal axons were closely associated with cells expressing R-cadherin message in the hypothalamus, the pretectum, and the anterolateral optic tectum. These results demonstrate that R-cadherin is appropriately distributed to play a role in regulating development of the zebrafish visual system, and in particular, pathfinding and synaptogenesis of retinal ganglion cell axons.

Requirement for the zebrafish mid-hindbrain boundary in midbrain polarisation, mapping and confinement of the retinotectal projection

The organizer at the midbrain-hindbrain boundary (MHB organizer) has been proposed to induce and polarize the midbrain during development. We investigate the requirement for the MHB organizer in acerebellar mutants, which lack a MHB and cerebellum, but retain a tectum, and are mutant for fgf8, a candidate inducer and polarizer. We examine the retinotectal projection in the mutants to assay polarity in the tectum. In mutant tecta, retinal ganglion cell (RGC) axons form overlapping termination fields, especially in the ventral tectum, and along both the anterior-posterior and dorsal-ventral axis of the tectum, consistent with a MHB requirement in generating midbrain polarity. However, polarity is not completely lost in the mutant tecta, in spite of the absence of the MHB. Moreover, graded expression of the ephrin family ligand Ephrin-A5b is eliminated, whereas Ephrin-A2 and Ephrin-A5a expression is leveled in acerebellar mutant tecta, showing that ephrins are differentially affected by the absence of the MHB. Some RGC axons overshoot beyond the mutant tectum, suggesting that the MHB also serves a barrier function for axonal growth. By transplanting whole eye primordia, we show that mapping defects and overshooting largely, but not exclusively, depend on tectal, but not retinal genotype, and thus demonstrate an independent function for Fgf8 in retinal development. The MHB organizer, possibly via Fgf8 itself, is thus required for midbrain polarisation and for restricting axonal growth, but other cell populations may also influence midbrain polarity.

FGF8 induces formation of an ectopic isthmic organizer and isthmocerebellar development via a repressive effect on Otx2 expression

Beads containing recombinant FGF8 (FGF8-beads) were implanted in the prospective caudal diencephalon or midbrain of chick embryos at stages 9–12. This induced the neuroepithelium rostral and caudal to the FGF8-bead to form two ectopic, mirror-image midbrains. Furthermore, cells in direct contact with the bead formed an outgrowth that protruded laterally from the neural tube. Tissue within such lateral outgrowths developed proximally into isthmic nuclei and distally into a cerebellum-like structure. These morphogenetic effects were apparently due to FGF8-mediated changes in gene expression in the vicinity of the bead, including a repressive effect on Otx2 and an inductive effect on En1, Fgf8 and Wnt1 expression. The ectopic Fgf8 and Wnt1 expression domains formed nearly complete concentric rings around the FGF8-bead, with the Wnt1 ring outermost. These observations suggest that FGF8 induces the formation of a ring-like ectopic signaling center (organizer) in the lateral wall of the brain, similar to the one that normally encircles the neural tube at the isthmic constriction, which is located at the boundary between the prospective midbrain and hindbrain. This ectopic isthmic organizer apparently sends long-range patterning signals both rostrally and caudally, resulting in the development of the two ectopic midbrains. Interestingly, our data suggest that these inductive signals spread readily in a caudal direction, but are inhibited from spreading rostrally across diencephalic neuromere boundaries. These results provide insights into the mechanism by which FGF8 induces an ectopic organizer and suggest that a negative feedback loop between Fgf8 and Otx2 plays a key role in patterning the midbrain and anterior hindbrain.

Engrailed negatively regulates the expression of cell adhesion molecules connectin and neuroglian in embryonic Drosophila nervous system

Engrailed is expressed in subsets of interneurons that do not express Connectin or appreciable Neuroglian, whereas other neurons that are Engrailed negative strongly express these adhesion molecules. Connectin and Neuroglian expression are virtually eliminated in interneurons when engrailed expression is driven ubiquitously in neurons, and greatly increased when engrailed genes are lacking in mutant embryos. The data suggest that Engrailed is normally a negative regulator of Connectin and neuroglian. These are the first two "effector" genes identified in the nervous system of Drosophila as regulatory targets for Engrailed. We argue that differential Engrailed expression is crucial in determining the pattern of expression of cell adhesion molecules and thus constitutes an important determinant of neuronal shape and perhaps connectivity.

The ephrins and Eph receptors in neural development

The Eph receptors are the largest known family of receptor tyrosine kinases. Initially all of them were identified as orphan receptors without known ligands, and their specific functions were not well understood. During the past few years, a corresponding family of ligands has been identified, called the ephrins, and specific functions have now been identified in neural development. The ephrins and Eph receptors are implicated as positional labels that may guide the development of neural topographic maps. They have also been implicated in pathway selection by axons, the guidance of cell migration, and the establishment of regional pattern in the nervous system. The ligands are anchored to cell surfaces, and most of the functions so far identified can be interpreted as precise guidance of cell or axon movement. This large family of ligands and receptors may make a major contribution to the accurate spatial patterning of connections and cell position in the nervous system.

Target selection: invasion, mapping and cell choice

Recent research has shown that changes in the concentration of particular molecules lead axons to invade their target, and that concentration changes in other molecules at the borders of the target prevent axons from leaving the target area. After invasion, topographic and lamina-specific cues guide axons to the correct location within the target field. At the level of a single cell or part of a cell, the evidence raises the possibility that axon targeting might be a combinatorial affair whereby specific axons compare the relative concentrations of several molecules on the surface of postsynaptic cells in order to choose a particular target. Both proteins and carbohydrates of various classes play major roles in these processes.

Expression and tyrosine phosphorylation of Eph receptors suggest multiple mechanisms in patterning of the visual system

The EphA3 receptor tyrosine kinase has been implicated in guiding the axons of retinal ganglion cells as they extend in the optic tectum. A repulsive mechanism involving opposing gradients of the EphA3 receptor on retinal axons and its ligands, ephrin-A2 and ephrin-A5, in the tectum influences topographic mapping of the retinotectal projection. To investigate the overall role of the Eph family in patterning of the visual system, we have used in situ hybridization to localize nine Eph receptors in the chicken retina and optic tectum at Embryonic Day 8. Three of the receptors examined correspond to the novel chicken homologs of EphA2, EphA6, and EphA7. Unexpectedly, we found that many Eph receptors are expressed not only in retinal ganglion cells, but also in tectal cells, In particular, EphA3 mRNA is prominently expressed in the anterior tectum, with a pattern reciprocal to that of ephrin-A2 and ephrin-A5. Similarly, ephrin-A5 is expressed not only in tectal cells but also in the nasal retina, with a pattern reciprocal to that of its receptor EphA3 and partially overlapping with that of its other receptor EphA4. Consistent with the even distribution of EphA4 and the polarized distribution of EphA4 ligands in the retina, probing EphA4 immunoprecipitates from different sectors of the retina with anti-phosphotyrosine antibodies revealed spatial differences in receptor phosphorylation. These complex patterns of expression and tyrosine phosphorylation suggest that Eph receptors and ephrins contribute to establishing topography of retinal axons through multiple mechanisms, in addition to playing a role in intraretinal and intratectal organization.

Specification of the anterior hindbrain and establishment of a normal mid/hindbrain organizer is dependent on Gbx2 gene function

Analysis of mouse embryos homozygous for a loss-of-function allele of Gbx2 demonstrates that this homeobox gene is required for normal development of the mid/hindbrain region. Gbx2 function appears to be necessary at the neural plate stage for the correct specification and normal proliferation or survival of anterior hindbrain precursors. It is also required to maintain normal patterns of expression at the mid/hindbrain boundary of Fgf8 and Wnt1, genes that encode signaling molecules thought to be key components of the mid/hindbrain (isthmic) organizer. In the absence of Gbx2 function, isthmic nuclei, the cerebellum, motor nerve V, and other derivatives of rhombomeres 1–3 fail to form. Additionally, the posterior midbrain in the mutant embryos appears to be extended caudally and displays abnormalities in anterior/posterior patterning. The failure of anterior hindbrain development is presumably due to the loss of Gbx2 function in the precursors of the anterior hindbrain. However, since Gbx2 expression is not detected in the midbrain it seems likely that the defects in midbrain anterior/posterior patterning result from an abnormal isthmic signaling center. These data provide genetic evidence for a link between patterning of the anterior hindbrain and the establishment of the mid/hindbrain organizer, and identify Gbx2 as a gene required for these processes to occur normally.

Pax-6 functions in boundary formation and axon guidance in the embryonic mouse forebrain

The Pax-6 gene encodes a transcription factor that is expressed in regionally restricted patterns in the developing brain and eye. Here we describe Pax-6 expression in the early forebrain (prosencephalon) on embryonic day 9.5 (E9.5) to E10.5 using both whole-mount in situ hybridization and antibody labeling. We find close correlations between Pax-6+ domains and initial neural patterning, and identify corresponding defects in embryos homozygous for the Pax-6 allele, Small eye (Sey). Pax-6 expression defines the prosencephalon-mesencephalon boundary, and mutant embryos lack this morphological boundary. Markers of the caudal prosencephalon are lost (Pax-6, Lim-1, Gsh-1) and a marker for mesencephalon is expanded rostrally into the prosencephalon (Dbx). We conclude that the caudal prosencephalon (prosomere 1) is at least partially transformed to a mesencephalic fate. This transformation results in a specific deficit of posterior commissure axons. Sey/Sey embryos also exhibit an axon pathfinding defect specific to the first longitudinal tract in the prosencephalon (tpoc, tract of the postoptic commissure). In wild type, tpoc axons fan out upon coming in contact with a superficial patch of Pax-6+ neuron cell bodies. In the mutant, the tpoc axons have normal initial projections, but make dramatic errors where they contact the neuron cell bodies, and fail to pioneer this first tract. Thus Pax-6 is required for local navigational information used by axons passing through its domain of expression. We conclude that Pax-6 plays multiple roles in forebrain patterning, including boundary formation, regional patterning, neuron specification and axon guidance.