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.

Retinoic acid signaling regulates proliferation and lamina formation in the developing chick optic tectum.

The retinotectal system has been extensively studied for investigating the mechanism(s) for topographic map formation. The optic tectum, which is composed of multiple laminae, is the major retino recipient structure in the developing avian brain. Laminar development of the tectum results from cell proliferation, differentiation and migration, coordinated in strict temporal and spatial patterns. However, the molecular mechanisms that orchestrate these complex developmental events, have not been fully elucidated. In this study, we have identified the presence of differential retinoic acid (RA) signaling along the rostro-caudal and dorsoventral axis of the tectum. We show for the first time that loss of RA signaling in the anterior optic tectum, leads to an increase in cell proliferation and gross changes in the morphology manifested as defects in lamination. Detailed analysis points to delayed migration of cells as the plausible cause for the defects in lamina formation. Thus, we conclude that in the optic tectum, RA signaling is involved in maintaining cell proliferation and in regulating the formation of the tectal laminae.

Microglia phagocytose myelin sheaths to modify developmental myelination.

During development, oligodendrocytes contact and wrap neuronal axons with myelin. Similarly to neurons and synapses, excess myelin sheaths are produced and selectively eliminated, but how elimination occurs is unknown. Microglia, the resident immune cells of the central nervous system, engulf surplus neurons and synapses. To determine whether microglia also prune myelin sheaths, we used zebrafish to visualize and manipulate interactions between microglia, oligodendrocytes, and neurons during development. We found that microglia closely associate with oligodendrocytes and specifically phagocytose myelin sheaths. By using a combination of optical, genetic, chemogenetic, and behavioral approaches, we reveal that neuronal activity bidirectionally balances microglial association with neuronal cell bodies and myelin phagocytosis in the optic tectum. Furthermore, multiple strategies to deplete microglia resulted in oligodendrocytes maintaining excessive and ectopic myelin. Our work reveals a neuronal activity-regulated role for microglia in modifying developmental myelin targeting by oligodendrocytes.

Regulation of axonal EphA4 forward signaling is involved in the effect of EphA3 on chicken retinal ganglion cell axon growth during retinotectal mapping.

The Eph and ephrins are involved in the genesis of topographic ordered connections at the visual system. Previously we demonstrated that tectal EphA3 stimulates axon growth of nasal retinal ganglion cells (RGCs) toward the caudal tectum preventing them from branching in the rostral tectum. Now we investigated whether tectal EphA3 plays this role by modulating the axonal EphA4 forward signaling or throughout axonal ephrin-As reverse signaling. For this purpose we used cultures of nasal retinal explants and dissociated retinal neurons from chicken embryos. We treated them with clustered EphA3-Fc, Fc (control), PI-PLC (sheds ephrin-As) or KYL (inhibits ephrin-As-mediated EphA4 activation). We achieved in vitro and in vivo electroporations of chicken embryo retinas with wild type EphA4, Ki-EphA4 (kinase inactive dominant negative EphA4) or EGFP in pMES expression vector. We performed immunocytochemistry, immunoprecipitation and Western blot against Eph/ephrin-As system. Our results showed that: 1) shedding of ephrin-As and the inhibition of ephrin-A-mediated EphA4 activity increase axon length and decrease axonal interstitial filopodia density of nasal RGCs; and 2) a dominant negative form of EphA4 increases axon growth in vitro and induces nasal RGC axons to grow passing throughout their target area in the caudal tectum meanwhile overexpression of EphA4 produces the opposite effects. All together, these results demonstrate that ephrin-A-mediated EphA4 forward signaling decreases the level of axon growth and increases the density of axonal interstitial filopodia of nasal RGCs. Besides, our results showed that: 3) EphA3 ectodomain increases axon growth and decreases the density of axonal interstitial filopodia and branching in vitro and in vivo and 4) EphA3 ectodomain diminishes the ephrin-A2/EphA4 colocalization, and the EphA4 and ephexin1 phosphorylation. All together, these results show that the EphA3 ectodomain produces the opposite effects than the EphA4 forward signaling, by decreasing this signaling pathway throughout competing with EphA4 for ephrin-As binding. Furthermore, it is proposed that tectal EphA3 participates in the establishment of retinotectal mapping throughout this mechanism and that EphAs can regulate axon growth and branching by modulating other EphA receptors forward signaling.

NGF, TrkA-P and neuroprotection after a hypoxic event in the developing central nervous system.

A decrease in the concentration of oxygen in the blood and tissues (hypoxia) produces important, sometimes irreversible, damages in the central nervous system (CNS) both during development and also postnatally. The present work aims at analyzing the expression of nerve growth factor (NGF) and p75 and the activation of TrkA in response to an acute normobaric hypoxic event and to evaluate the possible protective role of exogenous NGF. The developing chick optic tectum (OT), a recognized model of corticogenesis, was used as experimental system by means of in vivo and in vitro studies. Based on identification of the period of highest sensitivity of developmental programmed cell death (ED15) we show that hypoxia has a mild but reproducible effect that consist of a temporal increase of cell death 6 h after the end of a hypoxic treatment. Cell death was preceded by a significant early increase in the expression of Nerve Growth Factor (NGF) and its membrane receptor p75. In addition, we found a biphasic response of TrkA activation: a decrease during hypoxia followed by an increase -4 h later- that temporally coincide with the interval of NGF overexpression. To test the NGF - NGF receptors role in hypoxic cell death, we quantified, in primary neuronal cultures derived from ED15 OT, the levels of TrkA activation after an acute hypoxic treatment. A significant decline in the level of TrkA activation was observed during hypoxia followed, 24 h later, by significant cell death. Interestingly, this cell death can be reverted if TrkA inactivation during hypoxia is suppressed by the addition of NGF. Our results suggest that TrkA activation may play an important role in the survival of OT neurons subjected to acute hypoxia. The role of TrkA in neuronal survival after injury may be advantageously used for the generation of neuroprotective strategies to improve prenatal insult outcomes.

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

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

Effects of early eye removal on the morphology of a multisensory neuron in the chicken optic tectum.

The midbrain is a subcortical area involved in central functions such as integrating sensory modalities, movement initiation and bottom-up and top-down attention. In chicken, the midbrain roof is termed optic tectum (TeO) and consists of 15 layers with distinct in- and output regions. Visual input targets the superficial layers, while auditory input terminates in deeper layers. It has been shown that ablation of sensory epithelia leads to changes in the cellular patterning and structural organization of the sensory pathways. For the tectum, ablation of the eye anlagen was shown to affect retinorecipient neurons. While the gross morphology remained intact after enucleation, the shape of dendritic endings was changed presumably due to missing presynaptic input during synaptic pruning. We investigated the effect of deafferentation in a multisensory cell type, the Shepherd's crook neuron (SCN) in the TeO. SCNs have distinct dendritic branches in retinorecipient layers (superficial layers 1 to 5 and 7) and in layers where auditory input terminates. To assess whether removal of a single sensory input only affects the dendrites recipient for that input, we removed the eye anlagen and retrogradely labeled SCNs later in embryogenesis to visualize the morphology in lesioned and non-lesioned embryos. We found no changes in the gross morphology or in the basal dendrites, but an altered growth of the fine structures at the apical dendrite of SCNs in the retinorecipient layers. Our data indicate that the neuronal morphology of SCNs is mostly predefined before retinal innervation affect the fine structure.

Dispersing movement of tangential neuronal migration in superficial layers of the developing chick optic tectum.

During embryonic brain development, groups of particular neuronal cells migrate tangentially to participate in the formation of a laminated structure. Two distinct types of tangential migration in the middle and superficial layers have been reported in the development of the avian optic tectum. Here we show the dynamics of tangential cell movement in superficial layers of developing chick optic tectum. Confocal time-lapse microscopy in organotypic slice cultures and flat-mount cultures revealed that vigorous cell migration continued during E6.5-E13.5, where horizontally elongated superficial cells spread out tangentially. Motile cells exhibited exploratory behavior in reforming the branched leading processes to determine their pathway, and intersected with each other for dispersion. At the tectal peripheral border, the cells retraced or turned around to avoid protruding over the border. The tangentially migrating cells were eventually distributed in the outer stratum griseum et fibrosum superficiale and differentiated into neurons of various morphologies. These results revealed the cellular dynamics for widespread neuronal distribution in the superficial layers of the developing optic tectum, which underline a mode of novel tangential neuronal migration in the developing brain.

Analysis of experience-regulated transcriptome and imprintome during critical periods of mouse visual system development reveals spatiotemporal dynamics.

Visual system development is light-experience dependent, which strongly implicates epigenetic mechanisms in light-regulated maturation. Among many epigenetic processes, genomic imprinting is an epigenetic mechanism through which monoallelic gene expression occurs in a parent-of-origin-specific manner. It is unknown if genomic imprinting contributes to visual system development. We profiled the transcriptome and imprintome during critical periods of mouse visual system development under normal- and dark-rearing conditions using B6/CAST F1 hybrid mice. We identified experience-regulated, isoform-specific and brain-region-specific imprinted genes. We also found imprinted microRNAs were predominantly clustered into the Dlk1-Dio3 imprinted locus with light experience affecting some imprinted miRNA expression. Our findings provide the first comprehensive analysis of light-experience regulation of the transcriptome and imprintome during critical periods of visual system development. Our results may contribute to therapeutic strategies for visual impairments and circadian rhythm disorders resulting from a dysfunctional imprintome.

Sonic Hedgehog Regulation of the Neural Precursor Cell Fate During Chicken Optic Tectum Development.

During nervous system development, neurons project axons over long distances to reach the appropriate targets for correct neural circuit formation. Sonic hedgehog (Shh) is a secreted protein and plays a key role in regulating vertebrate embryogenesis, especially in central nervous system (CNS) patterning, including neuronal migration and axonal projection in the brain and spinal cord. In the developing ventral midbrain, Shh is sufficient to specify a striped pattern of cell fates. Little is known about the molecular mechanisms underlying the Shh regulation of the neural precursor cell fate during the optic tectum development. Here, we aimed at studying how Shh might regulate chicken optic tectum patterning. In the present study, in ovo electroporation methods were employed to achieve the overexpression of Shh in the optic tectum during chicken embryo development. Besides, the study combined in ovo electroporation and neuron isolation culturing to study the function of Shh in vivo and in vitro. The fluorescent immunohistochemistry methods were used to check the related indicators. The results showed that Shh overexpression caused 87.8% of cells to be distributed to the stratum griseum central (SGC) layer, while only 39.3% of the GFP-transfected cells resided in the SGC layer in the control group. Shh overexpression also reduced the axon length in vivo and in vitro. In conclusion, we provide evidence that Shh regulates the neural precursor cell fate during chicken optic tectum development. Shh overexpression impairs neuronal migration and may affect the fate determination of transfected neurons.

Deficiency of the Thyroid Hormone Transporter Monocarboxylate Transporter 8 in Neural Progenitors Impairs Cellular Processes Crucial for Early Corticogenesis.

Thyroid hormones (THs) are essential for establishing layered brain structures, a process called corticogenesis, by acting on transcriptional activity of numerous genes. In humans, deficiency of the monocarboxylate transporter 8 (MCT8), involved in cellular uptake of THs before their action, results in severe neurological abnormalities, known as the Allan-Herndon-Dudley syndrome. While the brain lesions predominantly originate prenatally, it remains unclear how and when exactly MCT8 dysfunction affects cellular processes crucial for corticogenesis. We investigated this by inducing in vivo RNAi vector-based knockdown of MCT8 in neural progenitors of the chicken optic tectum, a layered structure that shares many developmental features with the mammalian cerebral cortex. MCT8 knockdown resulted in cellular hypoplasia and a thinner optic tectum. This could be traced back to disrupted cell-cycle kinetics and a premature shift to asymmetric cell divisions impairing progenitor cell pool expansion. Birth-dating experiments confirmed diminished neurogenesis in the MCT8-deficient cell population as well as aberrant migration of both early-born and late-born neuroblasts, which could be linked to reduced reelin signaling and disorganized radial glial cell fibers. Impaired neurogenesis resulted in a reduced number of glutamatergic and GABAergic neurons, but the latter additionally showed decreased differentiation. Moreover, an accompanying reduction in untransfected GABAergic neurons suggests hampered intercellular communication. These results indicate that MCT8-dependent TH uptake in the neural progenitors is essential for early events in corticogenesis, and help to understand the origin of the problems in cortical development and function in Allan-Herndon-Dudley syndrome patients.SIGNIFICANCE STATEMENT Thyroid hormones (THs) are essential to establish the stereotypical layered structure of the human forebrain during embryonic development. Before their action on gene expression, THs require cellular uptake, a process facilitated by the TH transporter monocarboxylate transporter 8 (MCT8). We investigated how and when dysfunctional MCT8 can induce brain lesions associated with the Allan-Herndon-Dudley syndrome, characterized by psychomotor retardation. We used the layered chicken optic tectum to model cortical development, and induced MCT8 deficiency in neural progenitors. Impaired cell proliferation, migration, and differentiation resulted in an underdeveloped optic tectum and a severe reduction in nerve cells. Our data underline the need for MCT8-dependent TH uptake in neural progenitors and stress the importance of local TH action in early development.

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

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

Embryonic origin and lineage hierarchies of the neural progenitor subtypes building the zebrafish adult midbrain.

Neurogenesis in the post-embryonic vertebrate brain varies in extent and efficiency between species and brain territories. Distinct neurogenesis modes may account for this diversity, and several neural progenitor subtypes, radial glial cells (RG) and neuroepithelial progenitors (NE), have been identified in the adult zebrafish brain. The neurogenic sequences issued from these progenitors, and their contribution to brain construction, remain incompletely understood. Here we use genetic tracing techniques based on conditional Cre recombination and Tet-On neuronal birthdating to unravel the neurogenic sequence operating from NE progenitors in the zebrafish post-embryonic optic tectum. We reveal that a subpopulation of her5-positive NE cells of the posterior midbrain layer stands at the top of a neurogenic hierarchy involving, in order, the amplification pool of the tectal proliferation zone (TPZ), followed by her4-positive RG cells with transient neurogenic activity. We further demonstrate that the adult her5-positive NE pool is issued in lineage from an identically located NE pool expressing the same gene in the embryonic neural tube. Finally, we show that these features are reminiscent of the neurogenic sequence and embryonic origin of the her9-positive progenitor NE pool involved in the construction of the lateral pallium at post-embryonic stages. Together, our results highlight the shared recruitment of an identical neurogenic strategy by two remote brain territories, where long-lasting NE pools serve both as a growth zone and as the life-long source of young neurogenic RG cells.

Cryptochrome 1b: a possible inducer of visual lateralization in pigeons?

The visual system of adult pigeons shows a lateralization of object discrimination with a left hemispheric dominance on the behavioural, physiological and anatomical levels. The crucial trigger for the establishment of this asymmetry is the position of the embryo inside the egg, which exposes the right eye to light falling through the egg shell. As a result, the right-sided retina is more strongly stimulated with light during embryonic development. However, it is unknown how this embryonic light stimulation is transduced to the brain as rods and cones are not yet functional. A possible solution could be the blue-light-sensitive molecule cryptochrome 1 (Cry1), which is expressed in retinal ganglion cells (RGCs) of several mammalian and avian species. RGCs have been shown to be functional during the time of induction of asymmetry and possess projections to primary visual areas. Therefore, Cry1-containing RGCs could be responsible for induction of asymmetry. The aim of this study was to identify the expression pattern of the Cry1 subtype Cry1b in the retina of embryonic, post-hatch and adult pigeons by immunohistochemical staining and to show whether Cry1b-containing RGCs project to the optic tectum. Cry1b-positive cells were indeed mainly found in the RGC layer and to lesser extent in the inner nuclear layer at all ages, including the embryonic stage. Tracing in adult animals revealed that at least a subset of Cry1b-containing RGCs project to the optic tectum. Thus, Cry1b-containing RGCs within the embryonic retina could be involved in the induction of asymmetries in the visual system of pigeons.

Tangential cell migration during layer formation of chick optic tectum.

The laminated structure of the optic tectum is formed by radial and tangential cell migration during development. Studies of developing chick optic tectum have revealed two streams of tangential cell migration in the middle and superficial layers, which have distinctive origins, migratory paths, modes of migration, and destinations. We will review the process of the two types of tangential migrations, in order to elucidate their roles in the formation of the optic tectum layers.

Engrailed and tectum development.

The optic tectum is a visual center of nonmammalian vertebrates derived from the mesencephalon. In this review, function of Engrailed (En) in tectum development is reviewed. En plays crucial roles at three steps of tectum development. First, Engrailed is expressed in the mesencephalon and the metencephalon and essential for the regionalization of the mesencephalon. En is expressed in a gradient of caudal-to-rostral in the tectum primordial, and regulates the rostrocaudal polarity of the tectum. In the advanced stage of tectum development, En is expressed in a lamina-specific manner and it is suggested that En regulates cell migration in the tectal laminar formation.

The influence of activity on axon pathfinding in the optic tectum.

The relative importance of neural activity versus activity-independent cues in shaping the initial wiring of the brain is still largely an open question. While activity is clearly critical for circuit rearrangements after initial connections have been made, whether it also plays a role in initial axon pathfinding remains to be determined. Here, we investigated this question using the guidance of zebrafish retinal ganglion cell axons to their targets in the tectum as a model. Recent results have implicated biased branching as a key feature of pathfinding in the zebrafish tectum. Using tetrodotoxin to silence neural activity globally, we found a decrease in the area covered by axon branches during pathfinding. After reaching the target, there were dynamic differences in axon length, area and the number of branches between conditions. However, other aspects of pathfinding were unaffected by silencing, including the ratio of branches directed toward the target, length, and number of branches, as well as turning angle, velocity, and number of growth cones per axon. These results challenge the hypothesis that neural connections develop in sequential stages of molecularly guided pathfinding and activity-based refinement. Despite a maintenance of overall guidance, axon pathfinding dynamics can nevertheless be altered by activity loss.

Temporal-spatial correlation between angiogenesis and corticogenesis in the developing chick optic tectum.

The developing chick optic tectum is a widely used model of corticogenesis and angiogenesis. Cell behaviors involved in corticogenesis and angiogenesis share several regulatory mechanisms. In this way the 3D organizations of both systems adapt to each other. The consensus about the temporally and spatially organized progression of the optic tectum corticogenesis contrasts with the discrepancies about the spatial organization of its vascular bed as a function of the time. In order to find out spatial and temporal correlations between corticogenesis and angiogenesis, several methodological approaches were applied to analyze the dynamic of angiogenesis in the developing chick optic tectum. The present paper shows that a typical sequence of developmental events characterizes the optic tectum angiogenesis. The first phase, formation of the primitive vascular bed, takes place during the early stages of the tectal corticogenesis along which the large efferent neurons appear and begin their early differentiation. The second phase, remodeling and elaboration of the definitive vascular bed, occurs during the increase in complexity associated to the elaboration of the local circuit networks. The present results show that, apart from the well-known influence of the dorsal-ventral and radial axes as reference systems for the spatial organization of optic tectum angiogenesis, the cephalic-caudal axis also exerts a significant asymmetric influence. The term cortico-angiogenesis to describe the entire process is justified by the fact that tight correlations are found between specific corticogenic and angiogenic events and they take place simultaneously at the same position along the cephalic-caudal and radial axes.

Neuronal maturation and laminar formation in the chicken optic tectum are accompanied by the transition of phosphorylated cofilin from cytoplasm to nucleus.

Laminar formation in the chicken optic tectum requires processes that coordinate proliferation, migration and differentiation of neurons, in which the dynamics of actin filaments are crucial. Cofilin plays pivotal roles in regulating actin arrangement via its phosphorylation on Ser3. Given poor studies on the profile of phosphorylated cofilin (p-cofilin) in the developing tectum, we investigated its expression pattern. As determined by immunofluorescence histochemistry and western blotting, p-cofilin could be detected in most tectal layers except for the neural epithelium. In addition, we found p-cofilin was expressed both in the cytoplasm and the nucleus. During development, the expression of the cytoplasmic p-cofilin was decreasing and the nuclear p-cofilin was gradually increasing, but the total level of p-cofilin was down regulated. Double-labeling experiments revealed that the nuclear p-cofilin could be labeled in mature neurons but undetected in immature neurons. Furthermore, the number of cells co-stained with nuclear p-cofilin and NeuN was up-regulated during lamination and 60% cells were detected to be mature neurons that can express nuclear p-cofilin just at the first appearance of completed laminae. Our results demonstrate that the maturation of neurons is accompanied by this cytoplasm-to-nucleus transition of p-cofilin, and the nuclear p-cofilin can work effectively as a marker in the laminar formation of the chicken optic tectum.

NRP1-mediated Sema3A signals coordinate laminar formation in the developing chick optic tectum.

The optic tectum comprises multiple layers, which are formed by radial and tangential migration during development. Here, we report that Neuropilin 1 (NRP1)-mediated Sema3A signals are involved in the process of tectal laminar formation, which is elaborated by tangential migration. In the developing chick tectum, NRP1, a receptor for Sema3A, is expressed in microtubule-associated protein 2 (MAP2)-positive intermediate layers IV and V. Sema3A itself is a diffusible guidance factor and is expressed in the overlying layer VI. Using stable fluorescent labeling of tectal cells, we show that MAP2-positive intermediate layers are formed by the neurons that have been dispersed by tangential migration along the tectal efferent axons. When Sema3A was mis-expressed during laminar formation, local Sema3A repelled the tangential migrants, thus eliminating MAP2-positive neurons that expressed NRP1. Furthermore, in the absence of the MAP2-positive neurons, tectal layers were disorganized into an undulated form, indicating that MAP2-positive intermediate layers are required for proper laminar formation. These results suggest that NRP1-mediated Sema3A signals provide repulsive signals for MAP2-positive neurons to segregate tectal layers, which is important in order to coordinate laminar organization of the optic tectum.