Bone morphogenetic proteins, eye patterning, and retinocollicular map formation in the mouse

Transsynaptic BMP Signaling Regulates Fine-Scale Topography between Adjacent Sensory Neurons

Sensory axons projecting to the central nervous system are organized into topographic maps that represent the locations of sensory stimuli. In some sensory systems, even adjacent sensory axons are arranged topographically, forming "fine-scale" topographic maps. Although several broad molecular gradients are known to instruct coarse topography, we know little about the molecular signaling that regulates fine-scale topography at the level of two adjacent axons. Here, we provide evidence that transsynaptic bone morphogenetic protein (BMP) signaling mediates local interneuronal communication to regulate fine-scale topography in the nociceptive system of Drosophila larvae. We first show that the topographic separation of the axon terminals of adjacent nociceptors requires their common postsynaptic target, the A08n neurons. This phenotype is recapitulated by knockdown of the BMP ligand, Decapentaplegic (Dpp), in these neurons. In addition, removing the Type 2 BMP receptors or their effector (Mad transcription factor) in single nociceptors impairs the fine-scale topography, suggesting the contribution of BMP signaling originated from A08n. This signaling is likely mediated by phospho-Mad in the presynaptic terminals of nociceptors to ensure local interneuronal communication. Finally, reducing Dpp levels in A08n reduces the nociceptor-A08n synaptic contacts. Our data support that transsynaptic BMP signaling establishes the fine-scale topography by facilitating the formation of topographically correct synapses. Local BMP signaling for synapse formation may be a developmental strategy that independently regulates neighboring axon terminals for fine-scale topography.

Teneurin trans-axonal signaling prunes topographically missorted axons

Building precise neural circuits necessitates the elimination of axonal projections that have inaccurately formed during development. Although axonal pruning is a selective process, how it is initiated and controlled in vivo remains unclear. Here, we show that trans-axonal signaling mediated by the cell surface molecules Glypican-3, Teneurin-3, and Latrophilin-3 prunes misrouted retinal axons in the visual system. Retinotopic neuron transplantations revealed that pioneer ventral axons that elongate first along the optic tract instruct the pruning of dorsal axons that missort in that region. Glypican-3 and Teneurin-3 are both selectively expressed by ventral retinal ganglion cells and cooperate for correcting missorted dorsal axons. The adhesion G-protein-coupled receptor Latrophilin-3 signals along dorsal axons to initiate the elimination of topographic sorting errors. Altogether, our findings show an essential function for Glypican-3, Teneurin-3, and Latrophilin-3 in topographic tract organization and demonstrate that axonal pruning can be initiated by signaling among axons themselves.

Axon-Axon Interactions Regulate Topographic Optic Tract Sorting via CYFIP2-Dependent WAVE Complex Function

The axons of retinal ganglion cells (RGCs) are topographically sorted before they arrive at the optic tectum. This pre-target sorting, typical of axon tracts throughout the brain, is poorly understood. Here, we show that cytoplasmic FMR1-interacting proteins (CYFIPs) fulfill non-redundant functions in RGCs, with CYFIP1 mediating axon growth and CYFIP2 specifically involved in axon sorting. We find that CYFIP2 mediates homotypic and heterotypic contact-triggered fasciculation and repulsion responses between dorsal and ventral axons. CYFIP2 associates with transporting ribonucleoprotein particles in axons and regulates translation. Axon-axon contact stimulates CYFIP2 to move into growth cones where it joins the actin nucleating WAVE regulatory complex (WRC) in the periphery and regulates actin remodeling and filopodial dynamics. CYFIP2’s function in axon sorting is mediated by its binding to the WRC but not its translational regulation. Together, these findings uncover CYFIP2 as a key regulatory link between axon-axon interactions, filopodial dynamics, and optic tract sorting.

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CYFIP1 and CYFIP2 serve non-redundant functions in retinal axon growth and guidance

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CYFIP2 regulates growth cone filopodial dynamics and axon-axon responses

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CYFIP2 interacts with RNPs and the WRC in distinct cellular compartments

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Axon sorting is mediated by CYFIP2’s interaction with the WRC

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

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

Architecture, Function, and Assembly of the Mouse Visual System

Vision is the sense humans rely on most to navigate the world, make decisions, and perform complex tasks. Understanding how humans see thus represents one of the most fundamental and important goals of neuroscience. The use of the mouse as a model for parsing how vision works at a fundamental level started approximately a decade ago, ushered in by the mouse's convenient size, relatively low cost, and, above all, amenability to genetic perturbations. In the course of that effort, a large cadre of new and powerful tools for in vivo labeling, monitoring, and manipulation of neurons were applied to this species. As a consequence, a significant body of work now exists on the architecture, function, and development of mouse central visual pathways. Excitingly, much of that work includes causal testing of the role of specific cell types and circuits in visual perception and behavior-something rare to find in studies of the visual system of other species. Indeed, one could argue that more information is now available about the mouse visual system than any other sensory system, in any species, including humans. As such, the mouse visual system has become a platform for multilevel analysis of the mammalian central nervous system generally. Here we review the mouse visual system structure, function, and development literature and comment on the similarities and differences between the visual system of this and other model species. We also make it a point to highlight the aspects of mouse visual circuitry that remain opaque and that are in need of additional experimentation to enrich our understanding of how vision works on a broad scale.

Downstream mediators of Ten-m3 signalling in the developing visual pathway

Background

The formation of visuotopically-aligned projections in the brain is required for the generation of functional binocular circuits. The mechanisms which underlie this process are unknown. Ten-m3 is expressed in a broad high-ventral to low-dorsal gradient across the retina and in topographically-corresponding gradients in primary visual centres. Deletion of Ten-m3 causes profound disruption of binocular visual alignment and function. Surprisingly, one of the most apparent neuroanatomical changes—dramatic mismapping of ipsilateral, but not contralateral, retinal axons along the representation of the nasotemporal retinal axis—does not correlate well with Ten-m3’s expression pattern, raising questions regarding mechanism. The aim of this study was to further our understanding of the molecular interactions which enable the formation of functional binocular visual circuits.

Methods

Anterograde tracing, gene expression studies and protein pull-down experiments were performed. Statistical significance was tested using a Kolmogorov–Smirnov test, pairwise-fixed random reallocation tests and univariate ANOVAs.

Results

We show that the ipsilateral retinal axons in Ten-m3 knockout mice are mismapped as a consequence of early axonal guidance defects. The aberrant invasion of the ventral-most region of the dorsal lateral geniculate nucleus by ipsilateral retinal axons in Ten-m3 knockouts suggested changes in the expression of other axonal guidance molecules, particularly members of the EphA–ephrinA family. We identified a consistent down-regulation of EphA7, but none of the other EphA–ephrinA genes tested, as well as an up-regulation of ipsilateral-determinants Zic2 and EphB1 in visual structures. We also found that Zic2 binds specifically to the intracellular domain of Ten-m3 in vitro.

Conclusion

Our findings suggest that Zic2, EphB1 and EphA7 molecules may work as effectors of Ten-m3 signalling, acting together to enable the wiring of functional binocular visual circuits.

Electronic supplementary material

The online version of this article (10.1186/s12868-017-0397-5) contains supplementary material, which is available to authorized users.

Novel Models of Visual Topographic Map Alignment in the Superior Colliculus

The establishment of precise neuronal connectivity during development is critical for sensing the external environment and informing appropriate behavioral responses. In the visual system, many connections are organized topographically, which preserves the spatial order of the visual scene. The superior colliculus (SC) is a midbrain nucleus that integrates visual inputs from the retina and primary visual cortex (V1) to regulate goal-directed eye movements. In the SC, topographically organized inputs from the retina and V1 must be aligned to facilitate integration. Previously, we showed that retinal input instructs the alignment of V1 inputs in the SC in a manner dependent on spontaneous neuronal activity; however, the mechanism of activity-dependent instruction remains unclear. To begin to address this gap, we developed two novel computational models of visual map alignment in the SC that incorporate distinct activity-dependent components. First, a Correlational Model assumes that V1 inputs achieve alignment with established retinal inputs through simple correlative firing mechanisms. A second Integrational Model assumes that V1 inputs contribute to the firing of SC neurons during alignment. Both models accurately replicate in vivo findings in wild type, transgenic and combination mutant mouse models, suggesting either activity-dependent mechanism is plausible. In silico experiments reveal distinct behaviors in response to weakening retinal drive, providing insight into the nature of the system governing map alignment depending on the activity-dependent strategy utilized. Overall, we describe novel computational frameworks of visual map alignment that accurately model many aspects of the in vivo process and propose experiments to test them.

A transient wave of BMP signaling in the retina is necessary for Müller glial differentiation

The primary glial cells in the retina, the Müller glia, differentiate from retinal progenitors in the first postnatal week. CNTF/LIF/STAT3 signaling has been shown to promote their differentiation; however, another key glial differentiation signal, BMP, has not been examined during this period of Müller glial differentiation. In the course of our analysis of the BMP signaling pathway, we observed a transient wave of Smad1/5/8 signaling in the inner nuclear layer at the end of the first postnatal week, from postnatal day (P) 5 to P9, after the end of neurogenesis. To determine the function of this transient wave, we blocked BMP signaling during this period in vitro or in vivo, using either a BMP receptor antagonist or noggin (Nog). Either treatment leads to a reduction in expression of the Müller glia-specific genes Rlbp1 and Glul, and the failure of many of the Müller glia to repress the bipolar/photoreceptor gene Otx2. These changes in normal Müller glial differentiation result in permanent disruption of the retina, including defects in the outer limiting membrane, rosette formation and a reduction in functional acuity. Our results thus show that Müller glia require a transient BMP signal at the end of neurogenesis to fully repress the neural gene expression program and to promote glial gene expression.

Summary: BMP signalling is transiently activated in the postnatal mouse retina to terminate the neurogenic program and promote the expression of glial-specific genes.

Modeling development in retinal afferents: retinotopy, segregation, and ephrinA/EphA mutants

During neural development, neurons extend axons to target areas of the brain. Through processes of growth, branching and retraction these axons establish stereotypic patterns of connectivity. In the visual system, these patterns include retinotopic organization and the segregation of individual axons onto different subsets of target neurons based on the eye of origin (ocular dominance) or receptive field type (ON or OFF). Characteristic disruptions to these patterns occur when neural activity or guidance molecule expression is perturbed. In this paper we present a model that explains how these developmental patterns might emerge as a result of the coordinated growth and retraction of individual axons and synapses responding to position-specific markers, trophic factors and spontaneous neural activity. This model derives from one presented earlier (Godfrey et al., 2009) but which is here extended to account for a wider range of phenomena than previously described. These include ocular dominance and ON-OFF segregation and the results of altered ephrinA and EphA guidance molecule expression. The model takes into account molecular guidance factors, realistic patterns of spontaneous retinal wave activity, trophic molecules, homeostatic mechanisms, axon branching and retraction rules and intra-axonal signaling mechanisms that contribute to the survival of nearby synapses on an axon. We show that, collectively, these mechanisms can account for a wider range of phenomena than previous models of retino-tectal development.

Molecular guidance of retinotopic map development in the midbrain

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

Proteoglycan-mediated axon degeneration corrects pretarget topographic sorting errors

Proper arrangement of axonal projections into topographic maps is crucial for brain function, especially in sensory systems. An important mechanism for map formation is pretarget axon sorting, in which topographic ordering of axons appears in tracts before axons reach their target, but this process remains poorly understood. Here, we show that selective axon degeneration is used as a correction mechanism to eliminate missorted axons in the optic tract during retinotectal development in zebrafish. Retinal axons are not precisely ordered during initial pathfinding but become corrected later, with missorted axons selectively fragmenting and degenerating. We further show that heparan sulfate is required non-cell-autonomously to correct missorted axons and that restoring its synthesis at late stages in a deficient mutant is sufficient to restore topographic sorting. These findings uncover a function for developmental axon degeneration in ordering axonal projections and identify heparan sulfate as a key regulator of that process.

Identification of potential mediators of retinotopic mapping: a comparative proteomic analysis of optic nerve from WT and Phr1 retinal knockout mice

Retinal ganglion cells (RGCs) transmit visual information topographically from the eye to the brain, creating a map of visual space in retino-recipient nuclei (retinotopy). This process is affected by retinal activity and by activity-independent molecular cues. Phr1, which encodes a presumed E3 ubiquitin ligase (PHR1), is required presynaptically for proper placement of RGC axons in the lateral geniculate nucleus and the superior colliculus, suggesting that increased levels of PHR1 target proteins may be instructive for retinotopic mapping of retinofugal projections. To identify potential target proteins, we conducted a proteomic analysis of optic nerve to identify differentially abundant proteins in the presence or absence of Phr1 in RGCs. 1D gel electrophoresis identified a specific band in controls that was absent in mutants. Targeted proteomic analysis of this band demonstrated the presence of PHR1. Additionally, we conducted an unbiased proteomic analysis that identified 30 proteins as being significantly different between the two genotypes. One of these, heterogeneous nuclear ribonucleoprotein M (hnRNP-M), regulates antero-posterior patterning in invertebrates and can function as a cell surface adhesion receptor in vertebrates. Thus, we have demonstrated that network analysis of quantitative proteomic data is a useful approach for hypothesis generation and for identifying biologically relevant targets in genetically altered biological models.

Ten-m3 is required for the development of topography in the ipsilateral retinocollicular pathway

Background

The alignment of ipsilaterally and contralaterally projecting retinal axons that view the same part of visual space is fundamental to binocular vision. While much progress has been made regarding the mechanisms which regulate contralateral topography, very little is known of the mechanisms which regulate the mapping of ipsilateral axons such that they align with their contralateral counterparts.

Results

Using the advantageous model provided by the mouse retinocollicular pathway, we have performed anterograde tracing experiments which demonstrate that ipsilateral retinal axons begin to form terminal zones (TZs) in the superior colliculus (SC), within the first few postnatal days. These appear mature by postnatal day 11. Importantly, TZs formed by ipsilaterally-projecting retinal axons are spatially offset from those of contralaterally-projecting axons arising from the same retinotopic location from the outset. This pattern is consistent with that required for adult visuotopy. We further demonstrate that a member of the Ten-m/Odz/Teneurin family of homophilic transmembrane glycoproteins, Ten-m3, is an essential regulator of ipsilateral retinocollicular topography. Ten-m3 mRNA is expressed in a high-medial to low-lateral gradient in the developing SC. This corresponds topographically with its high-ventral to low-dorsal retinal gradient. In Ten-m3 knockout mice, contralateral ventrotemporal axons appropriately target rostromedial SC, whereas ipsilateral axons exhibit dramatic targeting errors along both the mediolateral and rostrocaudal axes of the SC, with a caudal shift of the primary TZ, as well as the formation of secondary, caudolaterally displaced TZs. In addition to these dramatic ipsilateral-specific mapping errors, both contralateral and ipsilateral retinocollicular TZs exhibit more subtle changes in morphology.

Conclusions

We conclude that important aspects of adult visuotopy are established via the differential sensitivity of ipsilateral and contralateral axons to intrinsic guidance cues. Further, we show that Ten-m3 plays a critical role in this process and is particularly important for the mapping of the ipsilateral retinocollicular pathway.

Extraocular ectoderm triggers dorsal retinal fate during optic vesicle evagination in zebrafish

Dorsal retinal fate is established early in eye development, via expression of spatially restricted dorsal-specific transcription factors in the optic vesicle; yet the events leading to initiation of dorsal fate are not clear. We hypothesized that induction of dorsal fate would require an extraocular signal arising from a neighboring tissue to pattern the prospective dorsal retina, however no such signal has been identified. We used the zebrafish embryo to determine the source, timing, and identity of the dorsal retina-inducing signal. Extensive cell movements occur during zebrafish optic vesicle morphogenesis, however the location of prospective dorsal cells within the early optic vesicle and their spatial relationship to early dorsal markers is currently unknown. Our mRNA expression and fate mapping analyses demonstrate that the dorsolateral optic vesicle is the earliest region to express dorsal specific markers, and cells from this domain contribute to the dorsal retinal pole at 24 hpf. We show that three bmp genes marking dorsal retina at 25 hpf are also expressed extraocularly before retinal patterning begins. We identified gdf6a as a dorsal initiation signal acting from the extraocular non-neural ectoderm during optic vesicle evagination. We find that bmp2b is involved in dorsal retina initiation, acting upstream of gdf6a. Together, this work has identified the nature and source of extraocular signals required to pattern the dorsal retina.

Eph and ephrin signaling in the formation of topographic maps

The axonal connections between the retina and its midbrain target, the superior colliculus (SC), is mapped topographically, such that the spatial relationships of cell bodies in the retina are maintained when terminating in the SC. Topographic map development uses a Cartesian mapping system such that each axis of the retina is mapped independently. Along the nasal-temporal mapping axis, EphAs and ephrin-As, are graded molecular cues required for topographic mapping while the dorsal-ventral axis is mapped in part via EphB and ephrin-Bs. Because both Ephs and ephrins are cell surface molecules they can signal in the forward and reverse directions. Eph/ephrin signaling leads to changes in cytoskeletal dynamics that lead to actin depolymerization and endocytosis guiding axons via attraction and repulsion.

Development of single retinofugal axon arbors in normal and β2 knock-out mice

The maturation of retinal ganglion cell (RGC) axon projections in the dorsal lateral geniculate nucleus (dLGN) and the superior colliculus (SC) relies on both molecular and activity-dependent mechanisms. Despite the increasing popularity of the mouse as a mammalian visual system model, little is known in this species about the normal development of individual RGC axon arbors or the role of activity in this process. We used a novel in vivo single RGC labeling technique to quantitatively characterize the elaboration and refinement of RGC axon arbors in the dLGN and SC in wild-type (WT) and β2-nicotinic acetylcholine receptors mutant (β2(-/-)) mice, which have perturbed retinal waves, during the developmental period when eye-specific lamination and retinotopic refinement occurs. Our results suggest that eye-specific segregation and retinotopic refinement in WT mice are not the result of refinement of richly exuberant arbors but rather the elaboration of arbors prepositioned in the proper location combined with the elimination of inappropriately targeted sparse branches. We found that retinocollicular arbors mature ∼1 week earlier than retinogeniculate arbors, although RGC axons reach the dLGN and SC at roughly the same age. We also observed striking differences between contralateral and ipsilateral RGC axon arbors in the SC but not in the LGN. These data suggest a strong influence of target specific cues during arbor maturation. In β2(-/-) mice, we found that retinofugal single axon arbors are well ramified but enlarged, particularly in the SC, indicating that activity-dependent visual map development occurs through the refinement of individual RGC arbors.

Smad4 is required predominantly in the developmental processes dependent on the BMP branch of the TGF-β signaling system in the embryonic mouse retina

PURPOSE

The present study was aimed at defining developmental roles of Smad4, a key mediator of the TGF-β superfamily signaling system, in the embryonic mouse retina.

METHODS

Using a Cre/loxP-mediated conditional gene targeting approach, Smad4 gene function was deleted from the embryonic mouse retina. Mutant phenotypes were morphologically and molecularly examined.

RESULTS

Loss of Smad4 in the developing retina led to varying degrees of microphthalmia at birth, presumably because of elevated apoptosis observed transiently at embryonic day 12.5 in the developing retina. This was also associated with an apparent delay in accumulation of retinal ganglion cells. Smad4 conditional mutants also exhibited alterations of retinal spatial patterning along the dorsal-ventral axis, consistent with a known function of BMP signaling in the embryonic retina. However, despite a known role for BMP signaling in retinal cell survival, proliferation, and differentiation, Smad4 mutant retinal progenitor cells were capable of maintaining growth and neurogenesis throughout embryonic development. We also found that the loss of Smad4 led to abnormal targeting of retinal ganglion cell axons to the optic nerve head, a phenotype consistent with reduced BMP signaling in the developing retina.

CONCLUSIONS

These results suggest that Smad4 is essential for a subset of, but not all, TGF-β/BMP-dependent developmental processes in the embryonic retina. In addition, genetic requirements for Smad4 in the embryonic retina are evident predominantly in the developmental events regulated by the BMP branch of the TGF-β signaling pathway.

Transfection of mouse retinal ganglion cells by in vivo electroporation

The targeting and refinement of RGC projections to the midbrain is a popular and powerful model system for studying how precise patterns of neural connectivity form during development. In mice, retinofugal projections are arranged in a topographic manner and form eye-specific layers in the Lateral Geniculate Nucleus (dLGN) of the thalamus and the Superior Colliculus (SC). The development of these precise patterns of retinofugal projections has typically been studied by labeling populations of RGCs with fluorescent dyes and tracers, such as horseradish peroxidase. However, these methods are too coarse to provide insight into developmental changes in individual RGC axonal arbor morphology that are the basis of retinotopic map formation. They also do not allow for the genetic manipulation of RGCs. Recently, electroporation has become an effective method for providing precise spatial and temporal control for delivery of charged molecules into the retina. Current retinal electroporation protocols do not allow for genetic manipulation and tracing of retinofugal projections of a single or small cluster of RGCs in postnatal mice. It has been argued that postnatal in vivo electroporation is not a viable method for transfecting RGCs since the labeling efficiency is extremely low and hence requires targeting at embryonic ages when RGC progenitors are undergoing differentiation and proliferation. In this video we describe an in vivo electroporation protocol for targeted delivery of genes, shRNA, and fluorescent dextrans to murine RGCs postnatally. This technique provides a cost effective, fast and relatively easy platform for efficient screening of candidate genes involved in several aspects of neural development including axon retraction, branching, lamination, regeneration and synapse formation at various stages of circuit development. In summary we describe here a valuable tool which will provide further insights into the molecular mechanisms underlying sensory map development.

Meis1 specifies positional information in the retina and tectum to organize the zebrafish visual system

Background

During visual system development, multiple signalling pathways cooperate to specify axial polarity within the retina and optic tectum. This information is required for the topographic mapping of retinal ganglion cell axons on the tectum. Meis1 is a TALE-class homeodomain transcription factor known to specify anterior-posterior identity in the hindbrain, but its role in visual system patterning has not been investigated.

Results

meis1 is expressed in both the presumptive retina and tectum. An analysis of retinal patterning reveals that Meis1 is required to correctly specify both dorsal-ventral and nasal-temporal identity in the zebrafish retina. Meis1-knockdown results in a loss of smad1 expression and an upregulation in follistatin expression, thereby causing lower levels of Bmp signalling and a partial ventralization of the retina. Additionally, Meis1-deficient embryos exhibit ectopic Fgf signalling in the developing retina and a corresponding loss of temporal identity. Meis1 also positively regulates ephrin gene expression in the tectum. Consistent with these patterning phenotypes, a knockdown of Meis1 ultimately results in retinotectal mapping defects.

Conclusions

In this work we describe a novel role for Meis1 in regulating Bmp signalling and in specifying temporal identity in the retina. By patterning both the retina and tectum, Meis1 plays an important role in establishing the retinotectal map and organizing the visual system.

ALCAM regulates mediolateral retinotopic mapping in the superior colliculus

ALCAM [activated leukocyte cell adhesion molecule (BEN/SC-1/DM-GRASP)] is a transmembrane recognition molecule of the Ig superfamily (IgSF) containing five Ig domains (two V-type, three C2-type). Although broadly expressed in the nervous and immune systems, few of its developmental functions have been elucidated. Because ALCAM has been suggested to interact with the IgSF adhesion molecule L1, a determinant of retinocollicular mapping, we hypothesized that ALCAM might direct topographic targeting to the superior colliculus (SC) by serving as a substrate within the SC for L1 on incoming retinal ganglion cell (RGC) axons. ALCAM was expressed in the SC during RGC axon targeting and on RGC axons as they formed the optic nerve; however, it was downregulated distally on RGC axons as they entered the SC. Axon tracing with DiI revealed pronounced mistargeting of RGC axons from the temporal retina half of ALCAM null mice to abnormally lateral sites in the contralateral SC, in which these axons formed multiple ectopic termination zones. ALCAM null mutant axons were specifically compromised in medial orientation of interstitial branches, which is known to require the ankyrin binding function of L1. As a substrate, ALCAM-Fc protein promoted L1-dependent attachment of acutely dissociated retinal cells and an L1-expressing, ALCAM-negative cell line, consistent with an ALCAM-L1 heterophilic molecular interaction. Together, these results suggest a model in which ALCAM in the SC interacts with L1 on RGC axons to promote medial extension of RGC axon branches important for mediolateral axon targeting in the formation of retinocollicular maps.

Eye development

The vertebrate eye comprises tissues from different embryonic origins: the lens and the cornea are derived from the surface ectoderm, but the retina and the epithelial layers of the iris and ciliary body are from the anterior neural plate. The timely action of transcription factors and inductive signals ensure the correct development of the different eye components. Establishing the genetic basis of eye defects in zebrafishes, mouse, and human has been an important tool for the detailed analysis of this complex process. A single eye field forms centrally within the anterior neural plate during gastrulation; it is characterized on the molecular level by the expression of "eye-field transcription factors." The single eye field is separated into two, forming the optic vesicle and later (under influence of the lens placode) the optic cup. The lens develops from the lens placode (surface ectoderm) under influence of the underlying optic vesicle. Pax6 acts in this phase as master control gene, and genes encoding cytoskeletal proteins, structural proteins, or membrane proteins become activated. The cornea forms from the surface ectoderm, and cells from the periocular mesenchyme migrate into the cornea giving rise for the future cornea stroma. Similarly, the iris and ciliary body form from the optic cup. The outer layer of the optic cup becomes the retinal pigmented epithelium, and the main part of the inner layer of the optic cup forms later the neural retina with six different types of cells including the photoreceptors. The retinal ganglion cells grow toward the optic stalk forming the optic nerve. This review describes the major molecular players and cellular processes during eye development as they are known from frogs, zebrafish, chick, and mice-showing also differences among species and missing links for future research. The relevance to human disorders is one of the major aspects covered throughout the review.

Gdf6a is required for the initiation of dorsal-ventral retinal patterning and lens development

Dorsal-ventral patterning of the vertebrate retina is essential for accurate topographic mapping of retinal ganglion cell (RGC) axons to visual processing centers. Bone morphogenetic protein (Bmp) growth factors regulate dorsal retinal identity in vertebrate models, but the developmental timing of this signaling and the relative roles of individual Bmps remain unclear. In this study, we investigate the functions of two zebrafish Bmps, Gdf6a and Bmp4, during initiation of dorsal retinal identity, and subsequently during lens differentiation. Knockdown of zebrafish Gdf6a blocks initiation of retinal Smad phosphorylation and dorsal marker expression, while knockdown of Bmp4 produces no discernable retinal phenotype. These data, combined with analyses of embryos ectopically expressing Bmps, demonstrate that Gdf6a is necessary and sufficient for initiation of dorsal retinal identity. We note a profound expansion of ventral retinal identity in gdf6a morphants, demonstrating that dorsal BMP signaling antagonizes ventral marker expression. Finally, we demonstrate a role for Gdf6a in non-neural ocular tissues. Knockdown of Gdf6a leads to defects in lens-specific gene expression, and when combined with Bmp signaling inhibitors, disrupts lens fiber cell differentiation. Taken together, these data indicate that Gdf6a initiates dorsal retinal patterning independent of Bmp4, and regulates lens differentiation.

Intrinsic patterning and experience-dependent mechanisms that generate eye-specific projections and binocular circuits in the visual pathway

A defining feature of the mammalian nervous system is its complex yet precise circuitry. The mechanisms which underlie the generation of neural connectivity are the topic of intense study in developmental neuroscience. The mammalian visual pathway demonstrates precise retinotopic organization in subcortical and cortical pathways, together with the alignment and matching of eye-specific projections, and sophisticated cortical circuitry that enables the extraction of features underlying vision. New approaches employing molecular-genetic analyses, transgenic mice, novel recombinant probes, and high-resolution imaging are contributing to rapid progress and a new synthesis in the field. These approaches are revealing the ways in which intrinsic patterning mechanisms act in concert with experience-dependent mechanisms to shape visual projections and circuits.

Phr1 regulates retinogeniculate targeting independent of activity and ephrin-A signalling

Proper functioning of the mammalian visual system requires that connections between the eyes and their central targets develop precisely. At birth, axons from the two eyes project to broad, overlapping regions of the dorsal-lateral geniculate nucleus (dLGN). In the adult, retinal axons segregate into distinct monocular regions at stereotyped locations within the dLGN. This process is driven by both molecular cues and activity-dependent synaptic competition. Here we demonstrate that Phr1, an evolutionarily conserved regulator of synapse formation and axon guidance, defines a novel molecular pathway required for proper localization of retinogeniculate projections. Following conditional excision of Phr1 in the retina, eye-specific domains within the dLGN are severely disturbed, despite normal spontaneous retinal wave activity and monocular segregation. Although layer placement is dramatically altered, Phr1 mutant retinal axons respond to ephrin-A in vitro. These findings indicate that Phr1 is a key presynaptic regulator of retinogeniculate layer placement independent of activity, segregation, or ephrin-A signaling.

Consequences of axon guidance defects on the development of retinotopic receptive fields in the mouse colliculus

Gradients of molecular factors pattern the developing retina and superior colliculus (SC) and guide retinal ganglion cell (RGC) axons to their appropriate central target perinatally. During and subsequent to this period, spontaneous waves of action potentials sweep across the retina, providing an instructive topographic signal based on the correlations of firing patterns of neighbouring RGCs. How these activity-independent and activity-dependent factors interact during retinotopic map formation remains unclear. A typical phenotype of mutant mice lacking genes for one or more RGC axon guidance molecules is the presence of topographically inappropriate projections or 'ectopic spots'. Here, we examine mice that lack functional bone morphogenetic protein receptors (BMPRs) in the retina. Retinal BMP controls the graded expression of RGC axon guidance molecules, resulting in some dorsal RGCs projecting ectopically to locations in the SC that normally receive input from ventral retina. We examine the consequences of this anatomical phenotype in vivo by studying the receptive field (RF) properties of neurons in the superficial SC. We observe a mixture of physiological phenotypes in BMPR mutant mice; notably we find some neurons with ectopic RFs displaced in elevation, corresponding to the observed anatomical defect. However, in a result not necessarily congruent with the presence of focal ectopic projections, some neurons have split, enlarged and patchy/distorted RFs. These results are consistent with the effects of spontaneous retinal waves acting upon a disrupted molecular template, and they place significant limits on the form of an activity-dependent learning rule for the development of retinocollicular projections.

Canonical Wnt signaling is required for the maintenance of dorsal retinal identity

Accurate retinotectal axon pathfinding depends upon the correct establishment of dorsal-ventral retinal polarity. We show that dorsal retinal gene expression is regulated by Wnt signaling in the dorsal retinal pigment epithelium (RPE). We find that a Wnt reporter transgene and Wnt pathway components are expressed in the dorsal RPE beginning at 14-16 hours post-fertilization. In the absence of Wnt signaling, tbx5 and Bmp genes initiate normal dorsal retinal expression but are not maintained. The expression of these genes is rescued by the downstream activation of Wnt signaling, and tbx5 is rescued by Bmp signaling. Furthermore, activation of Wnt signaling cannot rescue tbx5 in the absence of Bmp signaling, suggesting that Wnt signaling maintains dorsal retinal gene expression by regulating Bmp signaling. We present a model in which dorsal RPE-derived Wnt activity maintains the expression of Bmp ligands in the dorsal retina, thus coordinating the patterning of these two ocular tissues.

Mechanisms of response homeostasis during retinocollicular map formation

The mechanisms of Hebbian synaptic plasticity have been widely hypothesized to play a role in the activity-dependent development of neural circuits. However, these mechanisms are inherently unstable and would lead to the runaway excitation or depression of circuits if left unchecked. In the last decade, a number of elegant studies have demonstrated that homeostatic plasticity mechanisms exist to stabilize neural networks and maintain the constancy of neuronal output in response to changes in activity levels. These include synaptic scaling, sliding threshold models of synaptic plasticity, dynamic regulation of the number and strength of synapses, and bidirectional control of intrinsic excitability. Recently, we showed that the total synaptic input onto individual neurons of the mouse superior colliculus is preserved regardless of the size of their visual receptive fields, a phenomenon we term 'response homeostasis'. Here, we argue that regulating the capacity for synaptic plasticity and controlling the number and strength of retinocollicular inputs can preserve collicular neuron output, and we present evidence that changes in intrinsic excitability are not associated with response homeostasis. We also review findings from a number of different mutant mice and discuss whether and how different cellular mechanisms may underlie response homeostasis. Combined with other studies, our work reveals an important role for homeostatic mechanisms in regulating functional connectivity during the construction of receptive fields and the refinement of topographic maps.