The dorsal-ventral axis of the neural retina is divided into multiple domains of restricted gene expression which exhibit features of lineage compartments

Characteristic tetraspanin expression patterns mark various tissues during early Xenopus development

The tetraspanins (Tspans) constitute a family of cell surface proteins with four transmembrane domains. Tspans have been found on the plasma membrane and on exosomes of various organelles. Reports on the function of Tspans during the early development of Xenopus have mainly focused on the expression of uroplakins in gametes. Although the roles of extracellular vesicles (EVs) including exosomes have been actively analyzed in cancer research, the contribution of EVs to early development is not well understood. This is because the diffusivity of EVs is not compatible with a very strict developmental process. In this study, we analyzed members of the Tspan family in early development of Xenopus. Expression was prominent in specific organs such as the notochord, eye, cranial neural crest cells (CNCs), trunk neural crest cells, placodes, and somites. We overexpressed several combinations of Tspans in CNCs in vitro and in vivo. Changing the partner changed the distribution of fluorescent-labeled Tspans. Therefore, it is suggested that expression of multiple Tspans in a particular tissue might produce heterogeneity of intercellular communication, which has not yet been recognized.

BMP3 is a novel locus involved in the causality of ocular coloboma

Coloboma, a congenital disorder characterized by gaps in ocular tissues, is caused when the choroid fissure fails to close during embryonic development. Several loci have been associated with coloboma, but these represent less than 40% of those that are involved with this disease. Here, we describe a novel coloboma-causing locus, BMP3. Whole exome sequencing and Sanger sequencing of patients with coloboma identified three variants in BMP3, two of which are predicted to be disease causing. Consistent with this, bmp3 mutant zebrafish have aberrant fissure closure. bmp3 is expressed in the ventral head mesenchyme and regulates phosphorylated Smad3 in a population of cells adjacent to the choroid fissure. Furthermore, mutations in bmp3 sensitize embryos to Smad3 inhibitor treatment resulting in open choroid fissures. Micro CT scans and Alcian blue staining of zebrafish demonstrate that mutations in bmp3 cause midface hypoplasia, suggesting that bmp3 regulates cranial neural crest cells. Consistent with this, we see active Smad3 in a population of periocular neural crest cells, and bmp3 mutant zebrafish have reduced neural crest cells in the choroid fissure. Taken together, these data suggest that Bmp3 controls Smad3 phosphorylation in neural crest cells to regulate early craniofacial and ocular development.

OFF bipolar cell density varies by subtype, eccentricity, and along the dorsal ventral axis in the mouse retina

The neural retina is organized along central-peripheral, dorsal-ventral, and laminar planes. Cellular density and distributions vary along the central-peripheral and dorsal-ventral axis in species including primates, mice, fish, and birds. Differential distribution of cell types within the retina is associated with sensitivity to different types of damage that underpin major retinal diseases, including macular degeneration and glaucoma. Normal variation in retinal distribution remains unreported for multiple cell types in widely used research models, including mouse. Here we map the distribution of all known OFF bipolar cell (BC) populations and horizontal cells. We report significant variation in the distribution of OFF BC populations and horizontal cells along the dorsal-ventral and central-peripheral axes of the retina. Distribution patterns are much more pronounced for some populations of OFF BC cells than others and may correspond to the cell type's specialized functions.

Retinal organoids: a window into human retinal development

Retinal development and maturation are orchestrated by a series of interacting signalling networks that drive the morphogenetic transformation of the anterior developing brain. Studies in model organisms continue to elucidate these complex series of events. However, the human retina shows many differences from that of other organisms and the investigation of human eye development now benefits from stem cell-derived organoids. Retinal differentiation methods have progressed from simple 2D adherent cultures to self-organising micro-physiological systems. As models of development, these have collectively offered new insights into the previously unexplored early development of the human retina and informed our knowledge of the key cell fate decisions that govern the specification of light-sensitive photoreceptors. Although the developmental trajectories of other retinal cell types remain more elusive, the collation of omics datasets, combined with advanced culture methodology, will enable modelling of the intricate process of human retinogenesis and retinal disease in vitro.

Summary: Retinal organoid systems derived from human pluripotent stem cells are micro-physiological systems that offer new insights into previously unexplored human retina development.

Detailed analysis of chick optic fissure closure reveals Netrin-1 as an essential mediator of epithelial fusion

Epithelial fusion underlies many vital organogenic processes during embryogenesis. Disruptions to these cause a significant number of human birth defects, including ocular coloboma. We provide robust spatial-temporal staging and unique anatomical detail of optic fissure closure (OFC) in the embryonic chick, including evidence for roles of apoptosis and epithelial remodelling. We performed complementary transcriptomic profiling and show that Netrin-1 (NTN1) is precisely expressed in the chick fissure margin during fusion but is immediately downregulated after fusion. We further provide a combination of protein localisation and phenotypic evidence in chick, humans, mice and zebrafish that Netrin-1 has an evolutionarily conserved and essential requirement for OFC, and is likely to have an important role in palate fusion. Our data suggest that NTN1 is a strong candidate locus for human coloboma and other multi-system developmental fusion defects, and show that chick OFC is a powerful model for epithelial fusion research.

Our bodies are made of many different groups of cells, which are arranged into tissues that perform specific roles. As tissues form in the embryo they must adopt precise three-dimensional structures, depending on their position in the body. In many cases this involves two edges of tissue fusing together to prevent gaps being present in the final structure.

In individuals with a condition called ocular coloboma some of the tissues in the eyes fail to merge together correctly, leading to wide gaps that can severely affect vision. There are currently no treatments available for ocular coloboma and in over 70% of patients the cause of the defect is not known. Identifying new genes that control how tissues fuse may help researchers to find what causes this condition and multiple other tissue fusion defects, and establish whether these may be preventable in the future.

Much of what is currently known about how tissues fuse has come from studying mice and zebrafish embryos. Although the extensive genetic tools available in these ‘models’ have proved very useful, both offer only a limited time window for observing tissues as they fuse, and the regions involved are very small. Chick embryos, on the other hand, are much larger than mouse or zebrafish embryos and are easier to access from within their eggs. This led Hardy et al. to investigate whether the developing chick eye could be a more useful model for studying the precise details of how tissues merge.

Examining chick embryos revealed that tissues in the base of their eyes fuse between five and eight days after the egg had been fertilised, a comparatively long time compared to existing models. Also, many of the genes that Hardy et al. found switched on in chick eyes as the tissues merged had previously been identified as being essential for tissue fusion in humans. However, several new genes were also shown to be involved in the fusing process. For example, Netrin-1 was important for tissues to fuse in the eyes as well as in other regions of the developing embryo.

These findings demonstrate that the chick eye is an excellent new model system to study how tissues fuse in animals. Furthermore, the genes identified by Hardy et al. may help researchers to identify the genetic causes of ocular coloboma and other tissue fusion defects in humans.

An analysis of anterior segment development in the chicken eye

Precise anterior segment (AS) development in the vertebrate eye is essential for maintaining ocular health throughout life. Disruptions to genetic programs can lead to severe structural AS disorders at birth, while more subtle AS defects may disrupt the drainage of ocular fluids and cause dysregulation of intraocular pressure homeostasis, leading to progressive vision loss. To date, the mouse has served as the major model to study AS development and pathogenesis. Here we present an accurate histological atlas of chick AS formation throughout eye development, with a focus on the formation of drainage structures. We performed expression analyses for a panel of known AS disorder genes, and showed that chick PAX6 was localized to cells of neural retina and surface ectoderm derived structures, displaying remarkable similarity to the mouse. We provide a comparison to mouse and humans for chick AS developmental sequences and structures and confirm that AS development shares common features in all three species, although the main AS structures in the chick are developed prior to hatching. These features enable the unique experimental advantages inherent to chick embryos, and we therefore propose the chick as an appropriate additional model for AS development and disease.

Complement component C3aR constitutes a novel regulator for chick eye morphogenesis

Complement components have been implicated in a wide variety of functions including neurogenesis, proliferation, cell migration, differentiation, cancer, and more recently early development and regeneration. Following our initial observations indicating that C3a/C3aR signaling induces chick retina regeneration, we analyzed its role in chick eye morphogenesis. During eye development, the optic vesicle (OV) invaginates to generate a bilayer optic cup (OC) that gives rise to the retinal pigmented epithelium (RPE) and neural retina. We show by immunofluorescence staining that C3 and the receptor for C3a (the cleaved and active form of C3), C3aR, are present in chick embryos during eye morphogenesis in the OV and OC. Interestingly, C3aR is mainly localized in the nuclear compartment at the OC stage. Loss of function studies at the OV stage using morpholinos or a blocking antibody targeting the C3aR (anti-C3aR Ab), causes eye defects such as microphthalmia and defects in the ventral portion of the eye that result in coloboma. Such defects were not observed when C3aR was disrupted at the OC stage. Histological analysis demonstrated that microphthalmic eyes were unable to generate a normal optic stalk or a closed OC. The dorsal/ventral patterning defects were accompanied by an expansion of the ventral markers Pax2, cVax and retinoic acid synthesizing enzyme raldh-3 (aldh1a3) domains, an absence of the dorsal expression of Tbx5 and raldh-1 (aldh1a1) and a re-specification of the ventral RPE to neuroepithelium. In addition, the eyes showed overall decreased expression of Gli1 and a change in distribution of nuclear β-catenin, suggesting that Shh and Wnt pathways have been affected. Finally, we observed prominent cell death along with a decrease in proliferating cells, indicating that both processes contribute to the microphthalmic phenotype. Together our results show that C3aR is necessary for the proper morphogenesis of the OC. This is the first report implicating C3aR in eye development, revealing an unsuspected hitherto regulator for proper chick eye morphogenesis.

Fgf8 Expression and Degradation of Retinoic Acid Are Required for Patterning a High-Acuity Area in the Retina

Species that are highly reliant on their visual system have a specialized retinal area subserving high-acuity vision, e.g., the fovea in humans. Although of critical importance for our daily activities, little is known about the mechanisms driving the development of retinal high-acuity areas (HAAs). Using the chick as a model, we found a precise and dynamic expression pattern of fibroblast growth factor 8 (Fgf8) in the HAA anlage, which was regulated by enzymes that degrade retinoic acid (RA). Transient manipulation of RA signaling, or reduction of Fgf8 expression, disrupted several features of HAA patterning, including photoreceptor distribution, ganglion cell density, and organization of interneurons. Notably, patterned expression of RA signaling components was also found in humans, suggesting that RA also plays a role in setting up the human fovea.

Emergence of dorsal-ventral polarity in ESC-derived retinal tissue

We previously demonstrated that mouse embryonic stem cell (mESC)-derived retinal epithelium self-forms an optic cup-like structure. In the developing retina, the dorsal and ventral sides differ in terms of local gene expression and morphological features. This aspect has not yet been shown in vitro Here, we demonstrate that mESC-derived retinal tissue spontaneously acquires polarity reminiscent of the dorsal-ventral (D-V) patterning of the embryonic retina. Tbx5 and Vax2 were expressed in a mutually exclusive manner, as seen in vivo Three-dimensional morphometric analysis showed that the in vitro-formed optic cup often contains cleft structures resembling the embryonic optic fissure. To elucidate the mechanisms underlying the spontaneous D-V polarization of mESC-derived retina, we examined the effects of patterning factors, and found that endogenous BMP signaling plays a predominant role in the dorsal specification. Further analysis revealed that canonical Wnt signaling, which was spontaneously activated at the proximal region, acts upstream of BMP signaling for dorsal specification. These observations suggest that D-V polarity could be established within the self-formed retinal neuroepithelium by intrinsic mechanisms involving the spatiotemporal regulation of canonical Wnt and BMP signals.

Spatiotemporal Patterns of Tumor Occurrence in Children with Intraocular Retinoblastoma

PURPOSE

To accurately map the retinal area covered by tumor in a prospectively enrolled cohort of children diagnosed with retinoblastoma.

METHODS

Orbital MRI in 106 consecutive retinoblastoma patients (44 bilateral) was analyzed. For MRI-visible tumors, the polar angle and angle of eccentricity of points defining tumor perimeter on the retina were determined by triangulation from images in three orthogonal planes. The centroid of the mapped area was calculated to approximate tumor origin, and the location and cumulative tumor burden were analyzed in relation to mutation type (germline vs. somatic), tumor area, and patient age at diagnosis. Location of small tumors undetected by MRI was approximated with fundoscopic images.

RESULTS

Mapping was successful for 129 tumors in 91 eyes from 67 patients (39 bilateral, 43 germline mutation). Cumulative tumor burden was highest within the macula and posterior pole and was asymmetrically higher within the inferonasal periphery. Tumor incidence was lowest in the superotemporal periphery. Tumor location varied with age at diagnosis in a complex pattern. Tumor location was concentrated in the macula and superonasal periphery in patients <5.6 months, in the inferotemporal quadrant of the posterior pole in patients 5.6-8.8 months, in the inferonasal quadrant in patients 8.8-13.2 months, and in the nasal and superotemporal periphery in patients >13.2 months. The distribution of MRI-invisible tumors was consistent with the asymmetry of mapped tumors.

CONCLUSIONS

MRI-based mapping revealed a previously unrecognized pattern of retinoblastoma localization that evolves with age at diagnosis. The structured spatiotemporal distribution of tumors may provide valuable clues about cellular or molecular events associated with tumorigenesis in the developing retina.

Early divergence of central and peripheral neural retina precursors during vertebrate eye development

BACKGROUND

During development of the vertebrate eye, optic tissue is progressively compartmentalized into functionally distinct tissues. From the central to the peripheral optic cup, the original optic neuroepithelial tissue compartmentalizes, forming retina, ciliary body, and iris. The retina can be further sub-divided into peripheral and central compartments, where the central domain is specialized for higher visual acuity, having a higher ratio and density of cone photoreceptors in most species.

RESULTS

Classically, models depict a segregation of the early optic cup into only two domains, neural and non-neural. Recent studies, however, uncovered discrete precursors for central and peripheral retina in the optic vesicle, indicating that the neural retina cannot be considered as a single unit with homogeneous specification and development. Instead, central and peripheral retina may be subject to distinct developmental pathways that underlie their specialization.

CONCLUSIONS

This review focuses on lineage relationships in the retina and revisits the historical context for segregation of central and peripheral retina precursors before overt eye morphogenesis.

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Central and peripheral retina arise through distinct developmental paths

In the mature eye, three distinct tissue fates, retina, ciliary body, and iris, arrange with a strict linear organization along the central (back) to peripheral (front) axis. The establishment of this topographical relationship within the optic vesicle is not well understood. We use a targeted vital labeling strategy to test the derivation of mature eye tissues from the optic vesicle of the chick embryo. Fate mapping uncovers two distinct origins of the neural retina. Contrary to expectations, the central neural retina has a discrete origin within the posterior optic vesicle. The peripheral retina derives from the distal optic vesicle, sharing a common origin with more peripheral tissue fates. This study identifies for the first time two distinct retinal sub-domains, central and peripheral, which arise during embryogenesis. Identification of these discrete retinal compartments provides a framework for understanding functional and disease processes throughout retinal tissue.

A glimpse into dorso-ventral patterning of the Drosophila eye

During organogenesis in all multi-cellular organisms, axial patterning is required to transform a single layer organ primordium into a three-dimensional organ. The Drosophila eye model serves as an excellent model to study axial patterning. Dorso-ventral (DV) axis determination is the first lineage restriction event during axial patterning of the Drosophila eye. The early Drosophila eye primordium has a default ventral fate, and the dorsal eye fate is established by onset of dorsal selector gene pannier (pnr) expression in a group of cells on the dorsal eye margin. The boundary between dorsal and ventral compartments called the equator is the site for Notch (N) activation, which triggers cell proliferation and differentiation. This review will focus on (1) chronology of events during DV axis determination; (2) how early division of eye into dorsal and ventral compartments contributes towards the growth and patterning of the fly retina, and (3) functions of DV patterning genes.

Conditional expression of the TVA receptor allows clonal analysis of descendents from Cre-expressing progenitor cells

An understanding of the number and types of progeny produced by progenitor cells during development provides a foundation for studies of when and where cell fate determination takes place. Lineal relationships can be revealed by the identification of descendents of cells that express a recombinase, such as Cre or Flp. This method provides data concerning gene expression history, but does not provide clonal resolution among the descendents. An alternative method employs retroviral labeling, which permits the identification of clones, but does not allow for the tracking of gene expression history. Here we report a combination of these methods to circumvent each method's limitations. By employing the specificity of Cre expression, and by selecting only a subset of cells with a Cre history for retroviral infection, clones with a gene expression history can be labeled. The method utilizes a conditional allele of the avian tumor virus receptor A (TVA), which allows infection of mouse cells following Cre activity, with mammalian retroviral vectors pseudotyped with the ASLV-A envelope glycoprotein (EnvA). We quantified the efficiency and specificity of this system in vivo and in vitro. We also generated a series of retroviral vectors encoding a variety of histochemical and fluorescent reporter genes that enable the tracking of mixtures of clones, thus enabling better resolution of clonal boundaries. This method and new vectors can be used to further our understanding of the gene expression patterns of progenitor cells that make particular daughter cells, as well as provide a platform for manipulating identified subsets of developing cells.

Dorsal eye selector pannier (pnr) suppresses the eye fate to define dorsal margin of the Drosophila eye

Axial patterning is crucial for organogenesis. During Drosophila eye development, dorso-ventral (DV) axis determination is the first lineage restriction event. The eye primordium begins with a default ventral fate, on which the dorsal eye fate is established by expression of the GATA-1 transcription factor pannier (pnr). Earlier, it was suggested that loss of pnr function induces enlargement in the dorsal eye due to ectopic equator formation. Interestingly, we found that in addition to regulating DV patterning, pnr suppresses the eye fate by downregulating the core retinal determination genes eyes absent (eya), sine oculis (so) and dacshund (dac) to define the dorsal eye margin. We found that pnr acts downstream of Ey and affects the retinal determination pathway by suppressing eya. Further analysis of the "eye suppression" function of pnr revealed that this function is likely mediated through suppression of the homeotic gene teashirt (tsh) and is independent of homothorax (hth), a negative regulator of eye. Pnr expression is restricted to the peripodial membrane on the dorsal eye margin, which gives rise to head structures around the eye, and pnr is not expressed in the eye disc proper that forms the retina. Thus, pnr has dual function, during early developmental stages pnr is involved in axial patterning whereas later it promotes the head specific fate. These studies will help in understanding the developmental regulation of boundary formation of the eye field on the dorsal eye margin.

Progenitor cells of the rod-free area centralis originate in the anterior dorsal optic vesicle

Background

Nervous system development is dependent on early regional specification to create functionally distinct tissues within an initially undifferentiated zone. Within the retina, photoreceptors are topographically organized with rod free area centrales faithfully generated at the centre of gaze. How does the developing eye regulate this placement? Conventional wisdom indicates that the distal tip of the growing optic vesicle (OV) gives rise to the area centralis/fovea. Ectopic expression and ablation studies do not fully support this view, creating a controversy as to the origin of this region. In this study, the lineage of cells in the chicken OV was traced using DiI. The location of labelled cells was mapped onto the retina in relation to the rod-free zone at embryonic (E) 7 and E17.5. The ability to regenerate a rod free area after OV ablation was determined in conjunction with lineage tracing.

Results

Anterior OV gave rise to cells in nasal retina and posterior OV became temporal retina. The OV distal tip gave rise to cells above the optic nerve head. A dorsal and anterior region of the OV correlated with cells in the developing rod free area centralis. Only ablations including the dorsal anterior region gave rise to a retina lacking a rod free zone. DiI application after ablation indicated that cells movements were greater along the anterior/posterior axis compared with the dorsal/ventral axis.

Conclusion

Our data support the idea that the chicken rod free area centralis originates from cells located near, but not at the distal tip of the developing OV. Therefore, the hypothesis that the area centralis is derived from cells at the distal tip of the OV is not supported; rather, a region anterior and dorsal to the distal tip gives rise to the rod free region. When compared with other studies of retinal development, our results are supported on molecular, morphological and functional levels. Our data will lead to a better understanding of the mechanisms underlying the topographic organization of the retina, the origin of the rod free zone, and the general issue of compartmentalization of neural tissue before any indication of morphological differentiation.

Signaling "cross-talk" is integrated by transcription factors in the development of the anterior segment in the eye

Extracellular signaling "cross-talk" between tissues is an important requirement for development of many organs yet the underlying mechanisms generally remain poorly understood. The anterior segment of the eye, which is constructed from four embryonic lineages, provides a unique opportunity to genetically dissect developmental processes such as signaling "cross-talk" without fear of inducing lethality. In the current review, we summarize recent data showing that PITX2, a homeodomain transcription factor, integrates retinoic acid and canonical Wnt/beta-catenin signaling during anterior segment development. Because the requirements for retinoic acid signaling, canonical Wnt/beta-catenin signaling, and PITX2 are not unique to the eye, this newly identified pathway may have relevance elsewhere during development and in tissue homeostasis.

Dynamic coupling of pattern formation and morphogenesis in the developing vertebrate retina

During embryonic development, pattern formation must be tightly synchronized with tissue morphogenesis to coordinate the establishment of the spatial identities of cells with their movements. In the vertebrate retina, patterning along the dorsal-ventral and nasal-temporal (anterior-posterior) axes is required for correct spatial representation in the retinotectal map. However, it is unknown how specification of axial cell positions in the retina occurs during the complex process of early eye morphogenesis. Studying zebrafish embryos, we show that morphogenetic tissue rearrangements during eye evagination result in progenitor cells in the nasal half of the retina primordium being brought into proximity to the sources of three fibroblast growth factors, Fgf8/3/24, outside the eye. Triple-mutant analysis shows that this combined Fgf signal fully controls nasal retina identity by regulating the nasal transcription factor Foxg1. Surprisingly, nasal-temporal axis specification occurs very early along the dorsal-ventral axis of the evaginating eye. By in vivo imaging GFP-tagged retinal progenitor cells, we find that subsequent eye morphogenesis requires gradual tissue compaction in the nasal half and directed cell movements into the temporal half of the retina. Balancing these processes drives the progressive alignment of the nasal-temporal retina axis with the anterior-posterior body axis and is controlled by a feed-forward effect of Fgf signaling on Foxg1-mediated cell cohesion. Thus, the mechanistic coupling and dynamic synchronization of tissue patterning with morphogenetic cell behavior through Fgf signaling leads to the graded allocation of cell positional identity in the eye, underlying retinotectal map formation.

Consequences of lineage-specific gene loss on functional evolution of surviving paralogs: ALDH1A and retinoic acid signaling in vertebrate genomes

Genome duplications increase genetic diversity and may facilitate the evolution of gene subfunctions. Little attention, however, has focused on the evolutionary impact of lineage-specific gene loss. Here, we show that identifying lineage-specific gene loss after genome duplication is important for understanding the evolution of gene subfunctions in surviving paralogs and for improving functional connectivity among human and model organism genomes. We examine the general principles of gene loss following duplication, coupled with expression analysis of the retinaldehyde dehydrogenase Aldh1a gene family during retinoic acid signaling in eye development as a case study. Humans have three ALDH1A genes, but teleosts have just one or two. We used comparative genomics and conserved syntenies to identify loss of ohnologs (paralogs derived from genome duplication) and to clarify uncertain phylogenies. Analysis showed that Aldh1a1 and Aldh1a2 form a clade that is sister to Aldh1a3-related genes. Genome comparisons showed secondarily loss of aldh1a1 in teleosts, revealing that Aldh1a1 is not a tetrapod innovation and that aldh1a3 was recently lost in medaka, making it the first known vertebrate with a single aldh1a gene. Interestingly, results revealed asymmetric distribution of surviving ohnologs between co-orthologous teleost chromosome segments, suggesting that local genome architecture can influence ohnolog survival. We propose a model that reconstructs the chromosomal history of the Aldh1a family in the ancestral vertebrate genome, coupled with the evolution of gene functions in surviving Aldh1a ohnologs after R1, R2, and R3 genome duplications. Results provide evidence for early subfunctionalization and late subfunction-partitioning and suggest a mechanistic model based on altered regulation leading to heterochronic gene expression to explain the acquisition or modification of subfunctions by surviving ohnologs that preserve unaltered ancestral developmental programs in the face of gene loss.

Key roles of Ephs and ephrins in retinotectal topographic map formation

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

Molecular analysis of stem cells and their descendants during cell turnover and regeneration in the planarian Schmidtea mediterranea

In adult planarians, the replacement of cells lost to physiological turnover or injury is sustained by the proliferation and differentiation of stem cells known as neoblasts. Neoblast lineage relationships and the molecular changes that take place during differentiation into the appropriate cell types are poorly understood. Here we report the identification and characterization of a cohort of genes specifically expressed in neoblasts and their descendants. We find that genes with severely downregulated expression after irradiation molecularly define at least three discrete subpopulations of cells. Simultaneous BrdU labeling and in situ hybridization experiments in intact and regenerating animals indicate that these cell subpopulations are related by lineage. Our data demonstrate not only the ability to measure and study the in vivo population dynamics of adult stem cells during tissue homeostasis and regeneration, but also the utility of studies in planarians to broadly inform stem cell biology in adult organisms.

Expression of SPIG1 reveals development of a retinal ganglion cell subtype projecting to the medial terminal nucleus in the mouse

Visual information is transmitted to the brain by roughly a dozen distinct types of retinal ganglion cells (RGCs) defined by a characteristic morphology, physiology, and central projections. However, our understanding about how these parallel pathways develop is still in its infancy, because few molecular markers corresponding to individual RGC types are available. Previously, we reported a secretory protein, SPIG1 (clone name; D/Bsp120I #1), preferentially expressed in the dorsal region in the developing chick retina. Here, we generated knock-in mice to visualize SPIG1-expressing cells with green fluorescent protein. We found that the mouse retina is subdivided into two distinct domains for SPIG1 expression and SPIG1 effectively marks a unique subtype of the retinal ganglion cells during the neonatal period. SPIG1-positive RGCs in the dorsotemporal domain project to the dorsal lateral geniculate nucleus (dLGN), superior colliculus, and accessory optic system (AOS). In contrast, in the remaining region, here named the pan-ventronasal domain, SPIG1-positive cells form a regular mosaic and project exclusively to the medial terminal nucleus (MTN) of the AOS that mediates the optokinetic nystagmus as early as P1. Their dendrites costratify with ON cholinergic amacrine strata in the inner plexiform layer as early as P3. These findings suggest that these SPIG1-positive cells are the ON direction selective ganglion cells (DSGCs). Moreover, the MTN-projecting cells in the pan-ventronasal domain are apparently composed of two distinct but interdependent regular mosaics depending on the presence or absence of SPIG1, indicating that they comprise two functionally distinct subtypes of the ON DSGCs. The formation of the regular mosaic appears to be commenced at the end of the prenatal stage and completed through the peak period of the cell death at P6. SPIG1 will thus serve as a useful molecular marker for future studies on the development and function of ON DSGCs.

Molecular mechanisms of vertebrate retina development: Implications for ganglion cell and photoreceptor patterning

Although the neural retina appears as a relatively uniform tissue when viewed from its surface, it is in fact highly patterned along its anterior-posterior and dorso-ventral axes. The question of how and when such patterns arise has been the subject of intensive investigations over several decades. Most studies aimed at understanding retinal pattern formation have used the retinotectal map, the ordered projections of retinal ganglion cells to the brain, as a functional readout of the pattern. However, other cell types are also topographically organized in the retina. The most commonly recognized example of such a topographic cellular organization is the differential distribution of photoreceptor types across the retina. Photoreceptor patterns are highly species-specific and may represent an important adaptation to the visual niche a given species occupies. Nevertheless, few studies have addressed this functional readout of pattern to date and our understanding of its development has remained superficial. Here, we review recent advances in understanding the molecular cascades that control regionalization of the eye anlage, relate these findings to the development of photoreceptor patterns and discuss common and unique strategies involved in both aspects of retinal pattern formation.

Retinal progenitor cells can produce restricted subsets of horizontal cells

Retinal progenitor cells have been shown to be multipotent throughout development. Similarly, many other structures of the developing central nervous system have been found to contain multipotent progenitor cells. Previous lineage studies did not address whether these multipotent progenitor cells were biased in their production of neuronal subtypes. This question is of interest because subtypes are the basis of distinct types of circuits. Here, lentivirus-mediated gene transfer was used to mark single retinal progenitor cells in vivo, and the different subtypes of horizontal cells (HCs) in each clone were quantified. Clones with two HCs consistently contained a single HC subtype, a pair of either H1 or H3 cells. This suggests that a multipotent progenitor cell produces a mitotic cell fated to make a terminal division that produces two HCs of only one subtype. This bias in production of one HC subtype suggests a previously undescribed mechanism of cell fate determination in at least a subset of retinal cells that involves decisions made by mitotic cells that are inherited in a symmetric manner by both neuronal daughter cells.

Molecular mechanisms of optic vesicle development: complexities, ambiguities and controversies

Optic vesicle formation, transformation into an optic cup and integration with neighboring tissues are essential for normal eye formation, and involve the coordinated occurrence of complex cellular and molecular events. Perhaps not surprisingly, these complex phenomena have provided fertile ground for controversial and even contradictory results and conclusions. After presenting an overview of current knowledge of optic vesicle development, we will address conceptual and methodological issues that complicate research in this field. This will be done through a review of the pertinent literature, as well as by drawing on our own experience, gathered through recent studies of both intra- and extra-cellular regulation of optic vesicle development and patterning. Finally, and without attempting to be exhaustive, we will point out some important aspects of optic vesicle development that have not yet received enough attention.

The level of BMP4 signaling is critical for the regulation of distinct T-box gene expression domains and growth along the dorso-ventral axis of the optic cup

BACKGROUND

Polarised gene expression is thought to lead to the graded distribution of signaling molecules providing a patterning mechanism across the embryonic eye. Bone morphogenetic protein 4 (Bmp4) is expressed in the dorsal optic vesicle as it transforms into the optic cup. Bmp4 deletions in human and mouse result in failure of eye development, but little attempt has been made to investigate mammalian targets of BMP4 signaling. In chick, retroviral gene overexpression studies indicate that Bmp4 activates the dorsally expressed Tbx5 gene, which represses ventrally expressed cVax. It is not known whether the Tbx5 related genes, Tbx2 and Tbx3, are BMP4 targets in the mammalian retina and whether BMP4 acts at a distance from its site of expression. Although it is established that Drosophila Dpp (homologue of vertebrate Bmp4) acts as a morphogen, there is little evidence that BMP4 gradients are interpreted to create domains of BMP4 target gene expression in the mouse.

RESULTS

Our data show that the level of BMP4 signaling is critical for the regulation of distinct Tbx2, Tbx3, Tbx5 and Vax2 gene expression domains along the dorso-ventral axis of the mouse optic cup. BMP4 signaling gradients were manipulated in whole mouse embryo cultures during optic cup development, by implantation of beads soaked in BMP4, or the BMP antagonist Noggin, to provide a local signaling source. Tbx2, Tbx3 and Tbx5, showed a differential response to alterations in the level of BMP4 along the entire dorso-ventral axis of the optic cup, suggesting that BMP4 acts across a distance. Increased levels of BMP4 caused expansion of Tbx2 and Tbx3, but not Tbx5, into the ventral retina and repression of the ventral marker Vax2. Conversely, Noggin abolished Tbx5 expression but only shifted Tbx2 expression dorsally. Increased levels of BMP4 signaling caused decreased proliferation, reduced retinal volume and altered the shape of the optic cup.

CONCLUSION

Our findings suggest the existence of a dorsal-high, ventral-low BMP4 signaling gradient across which distinct domains of Tbx2, Tbx3, Tbx5 and Vax2 transcription factor gene expression are set up. Furthermore we show that the correct level of BMP4 signaling is critical for normal growth of the mammalian embryonic eye.

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

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

dILA neurons in the dorsal spinal cord are the product of terminal and non-terminal asymmetric progenitor cell divisions, and require Mash1 for their development

dILA and dILB neurons comprise the major neuronal subtypes generated in the dorsal spinal cord, and arise in a salt-and-pepper pattern from a broad progenitor domain that expresses the bHLH factor Mash1. In this domain,Mash1-positive and Mash1-negative cells intermingle. Using a Mash1GFP allele in mice, we show here that Mash1+ progenitors give rise to dILA and dILB neurons. Using retroviral tracing in the chick, we demonstrate that a single progenitor can give rise to a dILA and a dILB neuron, and that dILA neurons are the product of asymmetric progenitor cell divisions. In Mash1-null mutant mice, the development of dILA, but not of dILB neurons is impaired. We provide evidence that a dual function of Mash1 in neuronal differentiation and specification accounts for the observed changes in the mutant mice. Our data allow us to assign to Mash1 a function in asymmetric cell divisions, and indicate that the factor coordinates cell cycle exit and specification in the one daughter that gives rise to a dILA neuron.

Mechanisms of ventral patterning in the vertebrate nervous system

Dorsoventral patterning of the neural tube has a crucial role in shaping the functional organization of the CNS. It is well established that hedgehog signalling plays a key role in specifying ventral cell types throughout the neuroectoderm, and major progress has been made in elucidating how hedgehog signalling works in this ventral specification. In addition, other molecular pathways, including nodal, retinoic acid and fibroblast growth factor signalling, have been identified as important molecular cues for ventral patterning of the spinal cord, telencephalon and eye. Here, we discuss recent advances in this field, highlighting the emerging interplay of these signalling pathways in the molecular specification of ventral patterning at different rostrocaudal levels of the CNS.

The rod photoreceptor pattern is set at the optic vesicle stage and requires spatially restricted cVax expression

How and when positional identities in the neural retina are established have been addressed primarily with respect to the topographic projections of retinal ganglion cells onto their targets in the brain. Although retinotectal map formation is a prominent manifestation of retinal patterning, it is not the only one. Photoreceptor subtypes are arranged in distinct, species-specific patterns. The mechanisms used to establish photoreceptor patterns have been relatively unexplored at the mechanistic level. We performed ablations of the eye anlage in chickens and found that removal of the anterior or dorsal optic vesicle caused loss of the area centralis, which is a rod-free central area of the retina, and severely disorganized other aspects of the rod pattern. These observations indicate that the anteroposterior and dorsoventral distribution of rods is determined by the optic vesicle stage. To investigate the molecular mechanisms involved, the rod distribution was analyzed after viral misexpression of several patterning genes that were previously shown to be important in positional specification of retinal ganglion cells. Ectopic expression of FoxG1, SOHo1,or GH6 transcription factors expressed in the anterior optic vesicle and/or optic cup, respectively, did not affect the rod pattern. This pattern therefore appears to be specified by an activity acting before, or in parallel with, these factors. In contrast, misexpression of the ventrally restricted transcription factor, cVax, severely disturbed the rod pattern.

Retinoic acid regulates the expression of dorsoventral topographic guidance molecules in the chick retina

Asymmetric expression of several genes in the early eye anlagen is required for the dorsoventral (DV) and anteroposterior (AP) patterning of the retina. Some of these early patterning genes play a role in determining the graded expression of molecules that are needed to form the retinotectal map. The polarized expression of retinoic acid synthesizing and degrading enzymes along the DV axis in the retina leads to several zones of varied retinoic acid (RA)activity. This is suggestive of RA playing a role in DV patterning of the retina. A dominant-negative form of the retinoic acid receptor α(DNhRARα) was expressed in the chick retina to block RA activity. RA signaling was found to play a role in regulating the expression of EphB2,EphB3 and ephrin B2, three molecules whose graded expression in the retina along the DV axis is important for establishing the correct retinotectal map. Blocking RA signaling by misexpression of a RA degrading enzyme, Cyp26A1 recapitulated some but not all the effects of DNhRARα. It also was found that Vax, a ventrally expressed transcription factor that regulates the expression of the EphB and ephrin B molecules, functions upstream of, or in parallel to, RA. Expression of DNhRARα led to increased levels of RA-synthesizing enzymes and loss of RA-degrading enzymes. Activation of such compensatory mechanisms when RA activity is blocked suggests that RA homeostasis is very strictly regulated in the retina.

Retinoic acid-dependent eye morphogenesis is orchestrated by neural crest cells

Using genetic approaches in the mouse, we show that the primary target tissue of retinoic acid (RA) action during eye morphogenesis is not the retina nor the corneal ectoderm, which both express RA-synthesizing retinaldehyde dehydrogenases (RALDH1 and RALDH3), but the neural crest cell-derived periocular mesenchyme (POM), which is devoid of RALDH. In POM, the effects of the paracrine RA signal are mediated by the nuclear RA receptors heterodimers RXRalpha/RARbeta and RXRalpha/RARgamma. These heterodimers appear to control: (1) the remodeling of the POM through activation of Eya2-related apoptosis; (2) the expression of Foxc1 and Pitx2, which play crucial roles in anterior eye segment development; and (3) the growth of the ventral retina. We additionally show that RALDH1 and RALDH3 are the only enzymes that are required for RA synthesis in the eye region from E10.5 to E13.5, and that patterning of the dorsoventral axis of the retina does not require RA.

Extraocular dorsal signal affects the developmental fate of the optic vesicle and patterns the optic neuroepithelium

Dorsal-ventral (DV) specification in the early optic vesicle plays a crucial role in the proper development of the eye. To address the questions of how DV specification is determined and how it affects fate determination of the optic vesicle, isolated optic vesicles were cultured either in vitro or in ovo. The dorsal and ventral halves of the optic vesicle were fated to develop into retinal pigment epithelium (RPE) and neural retina, respectively, when they were separated from each other and cultured. In optic vesicles treated with collagenase to remove the surrounding tissues, the neuroepithelium gave rise to cRax expression but not Mitf, suggesting that surrounding tissues are necessary for RPE specification. This was also confirmed in in ovo explant cultures. Combination cultures of collagenase-treated optic vesicles with either the dorsal or ventral part of the head indicated that head-derived factors have an important role in the fate determination of the optic vesicle: in the optic vesicles co-cultured with the dorsal part of the head Mitf expression was induced in the neuroepithelium, while the ventral head portion did not have this effect. The dorsal head also suppressed Pax2 expression in the optic vesicle. These observations indicate that factors from the dorsal head portion have important roles in the establishment of DV polarity within the optic vesicle, which in turn induces the patterning and differentiation of the neural retina and pigment epithelium.

Genetic interaction of Lobe with its modifiers in dorsoventral patterning and growth of the Drosophila eye

Dorsoventral (DV) patterning is essential for growth of the Drosophila eye. Recent studies suggest that ventral is the default state of the early eye, which depends on Lobe (L) function, and that the dorsal fate is established later by the expression of the dorsal selector gene pannier (pnr). However, the mechanisms of regulatory interactions between L and dorsal genes are not well understood. For studying the mechanisms of DV patterning in the early eye disc, we performed a dominant modifier screen to identify additional genes that interact with L. The criterion of the dominant interaction was either enhancement or suppression of the L ventral eye loss phenotype. We identified 48 modifiers that correspond to 16 genes, which include fringe (fng), a gene involved in ventral eye patterning, and members of both Hedgehog (Hh) and Decapentaplegic (Dpp) signaling pathways, which promote L function in the ventral eye. Interestingly, 29% of the modifiers (6 enhancers and 9 suppressors) identified either are known to interact genetically with pnr or are members of the Wingless (Wg) pathway, which acts downstream from pnr. The detailed analysis of genetic interactions revealed that pnr and L mutually antagonize each other during second instar of larval development to restrict their functional domains in the eye. This time window coincides with the emergence of pnr expression in the eye. Our results suggest that L function is regulated by multiple signaling pathways and that the mutual antagonism between L and dorsal genes is crucial for balanced eye growth.

Pax6 Interacts with cVax and Tbx5 to Establish the Dorsoventral Boundary of the Developing Eye

Dorsoventral pattern formation of the optic cup is essential for vertebrate eye morphogenesis and retinotectal topographic mapping. Dorsal and ventral aspects of the eye are distinct at early stages of development; cVax homeodomain protein expression is confined to the ventral optic cup, whereas Tbx5 (T-box transcription factor) expression domain becomes restricted to the dorsal region. Misexpression of cVax or Tbx5 induces profound defects in eye morphology and abnormal visual projections. In the Pax6-/- mutant Tbx5 fails to be expressed, and Vax1 and -2 are abnormally present in the entire optic vesicle. During eye development Pax6 becomes expressed in a gradient at the optic cup stage due to the specific activation of a highly conserved intronic alpha enhancer in the Pax6 locus. We observed that the highest level of Pax6 in the optic cup corresponds to the boundary between non-overlapping cVax and Tbx5 territories. To further investigate how these transcription factors control the patterning of the eye, we overexpressed Pax6 in the chick optic cup (E2) using in ovo electroporation. We observed that overexpression of Pax6 extends the Tbx5 and Bmp4 domains but reduces the cVax expression domains in the E3 chick eye. This results in an abnormal eye phenotype at E4. In addition, we showed that cVax and Tbx5 interact with Pax6 and modulate in an opposite manner the activity of the Pax6 alpha enhancer. Moreover, the Pax6/cVax interaction inhibits the transactivation properties of Pax6. These results demonstrate that Pax6 together with cVax and Tbx5 mediate dorsoventral patterning of the eye.

Initial state of the Drosophila eye before dorsoventral specification is equivalent to ventral

Dorsoventral (DV) patterning is crucial for eye development in invertebrates and higher animals. DV lineage restriction is the primary event in undifferentiated early eye primordia of Drosophila. In Drosophila eye disc, a dorsal-specific GATA family transcription factor pannier (pnr) controls Iroquois-Complex (Iro-C) genes to establish the dorsal eye fate whereas Lobe (L), which is involved in controlling a Notch ligand Serrate (Ser), is specifically required for ventral growth. However, fate of eye disc cells before the onset of dorsal expression of pnr and Iro-C is not known. We show that L/Ser are expressed in entire early eye disc before the expression of pnr and Iro-C is initiated in late first instar dorsal eye margin cells. Our evidence suggests that during embryogenesis pnr activity is not essential for eye development. We present evidence that loss of L or Ser function prior to initiation of pnr expression results in elimination of the entire eye, whereas after the onset of pnr expression it results only in preferential loss of ventral half of eye. We demonstrate that dorsal eye disc cells also become L or Ser dependent when they are ventralized by removal of pnr or Iro-C gene function. Therefore, we propose that early state of the eye prior to DV lineage restriction is equivalent to ventral and requires L and Ser gene function.

Isolation of a novel cDNA enriched in the undifferentiated chick retina and lens

By using a differential screening strategy, we have identified a previously uncharacterised gene that is preferentially expressed in chick retinal precursor cells as well as in the anterior epithelial cells of the lens at early stages of development. The EURL transcript has an open reading frame of 293 amino acids and a 3' untranslated region of >850 nucleotides. The only recognizable feature in the EURL (early undifferentiated retina and lens) amino acid sequence is a putative coiled-coil domain located at the C-terminus of the protein. The human EURL orthologue maps to chromosome 21q21. Northern blot analysis, in situ hybridization of tissue sections, and whole-mount hybridization indicate elevated levels of EURL mRNA in the embryonic dorsal retina from stage 14 (day 2) to stage 18 (day 3). At day 3.5, when >90% of the retinal cells are in the proliferative stage, EURL transcripts are found primarily in the peripheral dorsal retina, i.e., the most undifferentiated part of the dorsal retina. EURL transcripts are also detected in the lens at stage 18 and remain abundant in the proliferating epithelial cells of the lens until at least day 11. The distribution pattern of EURL in the developing retina and lens suggest a role before the events leading to cell determination and differentiation.

Regulation of axial patterning of the retina and its topographic mapping in the brain

Topographic maps are a fundamental organizational feature of axonal connections in the brain. A prominent model for studying axial polarity and topographic map development is the vertebrate retina and its projection to the optic tectum (or superior colliculus). Linked processes are controlled by molecules that are graded along the axes of the retina and its target fields. Recent studies indicate that ephrin-As control the temporal-nasal mapping of the retina in the optic tectum/superior colliculus by regulating the topographically-specific interstitial branching of retinal axons along the anterior-posterior tectal axis. This branching is mediated by relative levels of EphA receptor repellent signaling. A major recent advance is the demonstration that EphB receptor forward signaling and ephrin-B reverse signaling mediate axon attraction to control dorsal-ventral retinal mapping along the lateral-medial tectal axis. In addition, several classes of regulatory proteins have been implicated in the control of the axial patterning of the retina, and its ultimate readout of topographic mapping.