Identification of genes expressed preferentially in the developing peripheral margin of the optic cup

SWI/SNF complexes are required for retinal pigmented epithelium differentiation and for the inhibition of cell proliferation and neural differentiation programs

During embryonic development, tissue-specific transcription factors and chromatin remodelers function together to ensure gradual, coordinated differentiation of multiple lineages. Here, we define this regulatory interplay in the developing retinal pigmented epithelium (RPE), a neuroectodermal lineage essential for the development, function and maintenance of the adjacent retina. We present a high-resolution spatial transcriptomic atlas of the developing mouse RPE and the adjacent ocular mesenchyme obtained by geographical position sequencing (Geo-seq) of a single developmental stage of the eye that encompasses young and more mature ocular progenitors. These transcriptomic data, available online, reveal the key transcription factors and their gene regulatory networks during RPE and ocular mesenchyme differentiation. Moreover, conditional inactivation followed by Geo-seq revealed that this differentiation program is dependent on the activity of SWI/SNF complexes, shown here to control the expression and activity of RPE transcription factors and, at the same time, inhibit neural progenitor and cell proliferation genes. The findings reveal the roles of the SWI/SNF complexes in controlling the intersection between RPE and neural cell fates and the coupling of cell-cycle exit and differentiation.

Unravelling the link between embryogenesis and adult stem cell potential in the ciliary marginal zone: A comparative study between mammals and teleost fish

The discovery and study of adult stem cells have revolutionized regenerative medicine by offering new opportunities for treating various medical conditions. Anamniote stem cells, which retain their full proliferative capacity and full differentiation range throughout their lifetime, harbour a greater potential compared to mammalian adult stem cells, which only exhibit limited stem cell potential. Therefore, understanding the mechanisms underlying these differences is of significant interest. In this review, we examine the similarities and differences of adult retinal stem cells in anamniotes and mammals, from their embryonic stages in the optic vesicle to their residence in the postembryonic retinal stem cell niche, the ciliary marginal zone located in the retinal periphery. In anamniotes, developing precursors of retinal stem cells are exposed to various environmental cues during their migration in the complex morphogenetic remodelling of the optic vesicle to the optic cup. In contrast, their mammalian counterparts in the retinal periphery are primarily instructed by neighbouring tissues once they are in place. We explore the distinct modes of optic cup morphogenesis in mammals and teleost fish and highlight molecular mechanisms governing morphogenesis and stem cells instruction. The review concludes with the molecular mechanisms of ciliary marginal zone formation and offers a perspective on the impact of comparative single cell transcriptomic studies to reveal the evolutionary similarities and differences.

Comparing the transcriptome of developing native and iPSC-derived mouse retinae by single cell RNA sequencing

We report the generation and analysis of single-cell RNA-Seq data (> 38,000 cells) from mouse native retinae and induced pluripotent stem cell (iPSC)-derived retinal organoids at four matched stages of development spanning the emergence of the major retinal cell types. We combine information from temporal sampling, visualization of 3D UMAP manifolds, pseudo-time and RNA velocity analyses, to show that iPSC-derived 3D retinal organoids broadly recapitulate the native developmental trajectories. However, we observe relaxation of spatial and temporal transcriptome control, premature emergence and dominance of photoreceptor precursor cells, and susceptibility of dynamically regulated pathways and transcription factors to culture conditions in retinal organoids. We demonstrate that genes causing human retinopathies are enriched in cell-type specifying genes and identify a subset of disease-causing genes with expression profiles that are highly conserved between human retinae and murine retinal organoids. This study provides a resource to the community that will be useful to assess and further improve protocols for ex vivo recapitulation and study of retinal development.

Molecular networks of the FOXP2 transcription factor in the brain

The discovery of the FOXP2 transcription factor, and its implication in a rare severe human speech and language disorder, has led to two decades of empirical studies focused on uncovering its roles in the brain using a range of in vitro and in vivo methods. Here, we discuss what we have learned about the regulation of FOXP2, its downstream effectors, and its modes of action as a transcription factor in brain development and function, providing an integrated overview of what is currently known about the critical molecular networks.

Single-Cell RNA Sequencing of Retina:New Looks for Gene Marker and Old Diseases

The retina is composed of 11 types of cells, including neurons, glial cells and vascular bed cells. It contains five types of neurons, each with specific physiological, morphological, and molecular definitions. Currently, single-cell RNA sequencing (sRNA-seq) is emerging as one of the most powerful tools to reveal the complexity of the retina. The continuous discovery of retina-related gene targets plays an important role in helping us understand the nature of diseases. The revelation of new cell subpopulations can focus the occurrence and development of diseases on specific biological activities of specific cells. In addition, sRNA-seq performs high-throughput sequencing analysis of epigenetics, transcriptome and genome at the single-cell level, with the advantages of high-throughput and high-resolution. In this paper, we systematically review the development history of sRNA-seq technology, and summarize the new subtypes of retinal cells and some specific gene markers discovered by this technology. The progress in the diagnosis of retinal related diseases is also discussed.

Meis homeobox genes control progenitor competence in the retina

The vertebrate eye is derived from the neuroepithelium, surface ectoderm, and extracellular mesenchyme. The neuroepithelium forms an optic cup in which the spatial separation of three domains is established, namely, the region of multipotent retinal progenitor cells (RPCs), the ciliary margin zone (CMZ)-which possesses both a neurogenic and nonneurogenic potential-and the optic disk (OD), the interface between the optic stalk and the neuroretina. Here, we show by genetic ablation in the developing optic cup that Meis1 and Meis2 homeobox genes function redundantly to maintain the retinal progenitor pool while they simultaneously suppress the expression of genes characteristic of CMZ and OD fates. Furthermore, we demonstrate that Meis transcription factors bind regulatory regions of RPC-, CMZ-, and OD-specific genes, thus providing a mechanistic insight into the Meis-dependent gene regulatory network. Our work uncovers the essential role of Meis1 and Meis2 as regulators of cell fate competence, which organize spatial territories in the vertebrate eye.

The transcription factor Maz is essential for normal eye development

Wnt/β-catenin signaling has an essential role in eye development. Faulty regulation of this pathway results in ocular malformations, owing to defects in cell-fate determination and differentiation. Herein, we show that disruption of Maz, the gene encoding Myc-associated zinc-finger transcription factor, produces developmental eye defects in mice and humans. Expression of key genes involved in the Wnt cascade, Sfrp2, Wnt2b and Fzd4, was significantly increased in mice with targeted inactivation of Maz, resulting in abnormal peripheral eye formation with reduced proliferation of the progenitor cells in the region. Paradoxically, the Wnt reporter TCF-Lef1 displayed a significant downregulation in Maz-deficient eyes. Molecular analysis indicates that Maz is necessary for the activation of the Wnt/β-catenin pathway and participates in the network controlling ciliary margin patterning. Copy-number variations and single-nucleotide variants of MAZ were identified in humans that result in abnormal ocular development. The data support MAZ as a key contributor to the eye comorbidities associated with chromosome 16p11.2 copy-number variants and as a transcriptional regulator of ocular development.

Localization of Tfap2β, Casq2, Penk, Zic1, and Zic3 Expression in the Developing Retina, Muscle, and Sclera of the Embryonic Mouse Eye

Optic development involves sequential interactions between several different tissue types, including the overlying ectoderm, adjacent mesoderm, and neural crest mesenchyme and the neuroectoderm. In an ongoing expression screen, we identified that Tfap2β, Casq2, Penk, Zic1, and Zic3 are expressed in unique cell types in and around the developing eye. Tfap2β, Zic1, and Zic3 are transcription factors, Casq2 is a calcium binding protein and Penk is a neurotransmitter. Tfap2β, Zic1, and Zic3 have reported roles in brain and craniofacial development, while Casq2 and Penk have unknown roles. These five genes are expressed in the major tissue types in the eye, including the muscles, nerves, cornea, and sclera. Penk expression is found in the sclera and perichondrium. At E12.5 and E15.5, the extra-ocular muscles express Casq2, the entire neural retina expresses Zic1, and Zic3 is expressed in the optic disk and lip of the optic cup. The expression of Tfap2β expanded from corneal epithelium to the neural retina between E12.5 to E15.5. These genes are expressed in similar domains as Hedgehog (Gli1, and Ptch1) and the Wnt (Lef1) pathways. The expression patterns of these five genes warrant further study to determine their role in eye morphogenesis.

Single-Cell RNA Sequencing of hESC-Derived 3D Retinal Organoids Reveals Novel Genes Regulating RPC Commitment in Early Human Retinogenesis

The development of the mammalian retina is a complicated process involving the generation of distinct types of neurons from retinal progenitor cells (RPCs) in a spatiotemporal-specific manner. The progression of RPCs during retinogenesis includes RPC proliferation, cell-fate commitment, and specific neuronal differentiation. In this study, by performing single-cell RNA sequencing of cells isolated from human embryonic stem cell (hESC)-derived 3D retinal organoids, we successfully deconstructed the temporal progression of RPCs during early human retinogenesis. We identified two distinctive subtypes of RPCs with unique molecular profiles, namely multipotent RPCs and neurogenic RPCs. We found that genes related to the Notch and Wnt signaling pathways, as well as chromatin remodeling, were dynamically regulated during RPC commitment. Interestingly, our analysis identified that CCND1, a G₁-phase cell-cycle regulator, was coexpressed with ASCL1 in a cell-cycle-independent manner. Temporally controlled overexpression of CCND1 in retinal organoids demonstrated a role for CCND1 in promoting early retinal neurogenesis. Together, our results revealed critical pathways and novel genes in early retinogenesis of humans.

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Fate transition occurring in RPC is concomitant with onset of retinal neurogenesis

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Molecular dynamics underlying RPC commitment are dissected

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CCND1 promotes retinal neurogenesis in a cell-cycle-independent manner

Patterning the Vertebrate Retina with Morphogenetic Signaling Pathways

The primordium of the vertebrate eye is composed of a pseudostratified and apparently homogeneous neuroepithelium, which folds inward to generate a bilayered optic cup. During these early morphogenetic events, the optic vesicle is patterned along three different axes-proximo-distal, dorso-ventral, and naso-temporal-and three major domains: the neural retina, the retinal pigment epithelium (RPE), and the optic stalk. These fundamental steps that enable the subsequent development of a functional eye, entail the precise coordination among genetic programs. These programs are driven by the interplay of signaling pathways and transcription factors, which progressively dictate how each tissue should evolve. Here, we discuss the contribution of the Hh, Wnt, FGF, and BMP signaling pathways to the early patterning of the retina. Comparative studies in different vertebrate species have shown that their morphogenetic activity is repetitively used to orchestrate the progressive specification of the eye with evolutionary conserved mechanisms that have been adapted to match the specific need of a given species.

Eye organogenesis: A hierarchical view of ocular development

This chapter provides an overview of the early developmental origins of six ocular tissues: the cornea, lens, ciliary body, iris, neural retina, and retina pigment epithelium. Many of these tissue types are concurrently specified and undergo a complex set of morphogenetic movements that facilitate their structural interconnection. Within the context of vertebrate eye organogenesis, we also discuss the genetic hierarchies of transcription factors and signaling pathways that regulate growth, patterning, cell type specification and differentiation.

Regulation of neuronal and photoreceptor cell differentiation by Wnt signaling from iris-derived stem/progenitor cells of the chick in flat vs. Matrigel-embedding cultures

Previously we developed a simple culture method of the iris tissues and reported novel properties of neural stem/progenitor-like cells in the iris tissues of the chick and pig. When the iris epithelium or connective tissue (stroma) was treated with dispase, embedded in Matrigel, and cultured, neuronal cells extended from the explants within 24 h of culture, and cells positively stained for photoreceptor cell markers were also observed within a few days of culturing. In ordinary flat tissue culture conditions, explants had the same differentiation properties to those in tissue environments. Previously, we suggested that iris neural stem/progenitor cells are simply suppressed from neuronal differentiation within tissue, and that separation from the tissue releases the cells from this suppression mechanism. Here, we examined whether Wnt signaling suppressed neuronal differentiation of iris tissue cells in tissue environments because the lens, which has direct contact with the iris, is a rich source of Wnt proteins. When the Wnt signaling activator 6-bromoindirubin-3'-oxime (BIO) was administered to Matrigel culture, neuronal differentiation was markedly suppressed, but cell proliferation was not affected. When Wnt signaling inhibitors, such as DKK-1 and IWR-1, were applied to the same culture, they did not have any effect on cell differentiation and proliferation. However, when the inhibitors were applied to flat tissue culture, cells with neural properties emerged. These results indicate that the interaction of iris tissue with neighboring tissues and the environment regulates the stemness nature of iris tissue cells, and that Wnt signaling is a major factor.

The peripheral eye: A neurogenic area with potential to treat retinal pathologies?

Numerous degenerative diseases affecting visual function, including glaucoma and retinitis pigmentosa, are produced by the loss of different types of retinal cells. Cell replacement therapy has emerged as a promising strategy for treating these and other retinal diseases. The retinal margin or ciliary body (CB) of mammals has been proposed as a potential source of cells to be used in degenerative conditions affecting the retina because it has been reported it might hold neurogenic potential beyond embryonic development. However, many aspects of the origin and biology of the CB are unknown and more recent experiments have challenged the capacity of CB cells to generate different types of retinal neurons. Here we review the most recent findings about the development of the marginal zone of the retina in different vertebrates and some of the mechanisms underlying the proliferative and neurogenic capacity of this fascinating region of the vertebrates eye. In addition, we performed experiments to isolate CB cells from the mouse retina, generated neurospheres and observed that they can be expanded with a proliferative ratio similar to neural stem cells. When induced to differentiate, cells derived from the CB neurospheres start to express early neural markers but, unlike embryonic stem cells, they are not able to fully differentiate in vitro or generate retinal organoids. Together with previous reports on the neurogenic capacity of CB cells, also reviewed here, our results contribute to the current knowledge about the potentiality of this peripheral region of the eye as a therapeutic source of functional retinal neurons in degenerative diseases.

An atlas of Wnt activity during embryogenesis in Xenopus tropicalis

Wnt proteins form a family of highly conserved secreted molecules that are critical mediators of cell-cell signaling during embryogenesis. Partial data on Wnt activity in different tissues and at different stages have been reported in frog embryos. Our objective here is to provide a coherent and detailed description of Wnt activity throughout embryo development. Using a transgenic Xenopus tropicalis line carrying a Wnt-responsive reporter sequence, we depict the spatial and temporal dynamics of canonical Wnt activity during embryogenesis. We provide a comprehensive series of in situ hybridization in whole-mount embryos and in cross-sections, from gastrula to tadpole stages, with special focus on neural tube, retina and neural crest cell development. This collection of patterns will thus constitute a valuable resource for developmental biologists to picture the dynamics of Wnt activity during development.

The Ciliary Margin Zone of the Mammalian Retina Generates Retinal Ganglion Cells

The retina of lower vertebrates grows continuously by integrating new neurons generated from progenitors in the ciliary margin zone (CMZ). Whether the mammalian CMZ provides the neural retina with retinal cells is controversial. Live imaging of embryonic retina expressing eGFP in the CMZ shows that cells migrate laterally from the CMZ to the neural retina where differentiated retinal ganglion cells (RGCs) reside. Because Cyclin D2, a cell-cycle regulator, is enriched in ventral CMZ, we analyzed Cyclin D2^-/- mice to test whether the CMZ is a source of retinal cells. Neurogenesis is diminished in Cyclin D2 mutants, leading to a reduction of RGCs in the ventral retina. In line with these findings, in the albino retina, the decreased production of ipsilateral RGCs is correlated with fewer Cyclin D2⁺ cells. Together, these results implicate the mammalian CMZ as a neurogenic site that produces RGCs and whose proper generation depends on Cyclin D2 activity.

Multimodal Regulation Orchestrates Normal and Complex Disease States in the Retina

Regulation of biological processes occurs through complex, synergistic mechanisms. In this study, we discovered the synergistic orchestration of multiple mechanisms regulating the normal and diseased state (age related macular degeneration, AMD) in the retina. We uncovered gene networks with overlapping feedback loops that are modulated by nuclear hormone receptors (NHR), miRNAs, and epigenetic factors. We utilized a comprehensive filtering and pathway analysis strategy comparing miRNA and microarray data between three mouse models and human donor eyes (normal and AMD). The mouse models lack key NHRS (Nr2e3, RORA) or epigenetic (Ezh2) factors. Fifty-four total miRNAs were differentially expressed, potentially targeting over 150 genes in 18 major representative networks including angiogenesis, metabolism, and immunity. We identified sixty-eight genes and 5 miRNAS directly regulated by NR2E3 and/or RORA. After a comprehensive analysis, we discovered multimodal regulation by miRNA, NHRs, and epigenetic factors of three miRNAs (miR-466, miR1187, and miR-710) and two genes (Ell2 and Entpd1) that are also associated with AMD. These studies provide insight into the complex, dynamic modulation of gene networks as well as their impact on human disease, and provide novel data for the development of innovative and more effective therapeutics.

Msx1-Positive Progenitors in the Retinal Ciliary Margin Give Rise to Both Neural and Non-neural Progenies in Mammals

In lower vertebrates, stem/progenitor cells located in a peripheral domain of the retina, called the ciliary margin zone (CMZ), cooperate with retinal domain progenitors to build the mature neural retina. In mammals, it is believed that the CMZ lacks neurogenic potential and that the retina develops from one pool of multipotent retinal progenitor cells (RPCs). Here we identify a population of Msx1-expressing progenitors in the mouse CMZ that is both molecularly and functionally distinct from RPCs. Using genetic lineage tracing, we report that Msx1 progenitors have unique developmental properties compared with RPCs. Msx1 lineages contain both neural retina and non-neural ciliary epithelial progenies and overall generate fewer photoreceptors than classical RPC lineages. Furthermore, we show that the endocytic adaptor protein Numb regulates the balance between neural and non-neural fates in Msx1 progenitors. These results uncover a population of CMZ progenitors, distinct from classical RPCs, that also contributes to mammalian retinogenesis.

Ipsilateral and Contralateral Retinal Ganglion Cells Express Distinct Genes during Decussation at the Optic Chiasm

The increasing availability of transcriptomic technologies within the last decade has facilitated high-throughput identification of gene expression differences that define distinct cell types as well as the molecular pathways that drive their specification. The retinal projection neurons, retinal ganglion cells (RGCs), can be categorized into distinct morphological and functional subtypes and by the laterality of their projections. Here, we present a method for purifying the sparse population of ipsilaterally projecting RGCs in mouse retina from their contralaterally projecting counterparts during embryonic development through rapid retrograde labeling followed by fluorescence-activated cell sorting. Through microarray analysis, we uncovered the distinct molecular signatures that define and distinguish ipsilateral and contralateral RGCs during the critical period of axonal outgrowth and decussation, with more than 300 genes differentially expressed within these two cell populations. Among the differentially expressed genes confirmed through in vivo expression validation, several genes that mark “immaturity” are expressed within postmitotic ipsilateral RGCs. Moreover, at least one complementary pair, Igf1 and Igfbp5, is upregulated in contralateral or ipsilateral RGCs, respectively, and may represent signaling pathways that determine ipsilateral versus contralateral RGC identity. Importantly, the cell cycle regulator cyclin D2 is highly expressed in peripheral ventral retina with a dynamic expression pattern that peaks during the period of ipsilateral RGC production. Thus, the molecular signatures of ipsilateral and contralateral RGCs and the mechanisms that regulate their differentiation are more diverse than previously expected.

WNT/β-Catenin Signaling in Vertebrate Eye Development

The vertebrate eye is a highly specialized sensory organ, which is derived from the anterior neural plate, head surface ectoderm, and neural crest-derived mesenchyme. The single central eye field, generated from the anterior neural plate, divides to give rise to the optic vesicle, which evaginates toward the head surface ectoderm. Subsequently, the surface ectoderm, in conjunction with the optic vesicle invaginates to form the lens vesicle and double-layered optic cup, respectively. This complex process is controlled by transcription factors and several intracellular and extracellular signaling pathways including WNT/β-catenin signaling. This signaling pathway plays an essential role in multiple developmental processes and has a profound effect on cell proliferation and cell fate determination. During eye development, the activity of WNT/β-catenin signaling is tightly controlled. Faulty regulation of WNT/β-catenin signaling results in multiple ocular malformations due to defects in the process of cell fate determination and differentiation. This mini-review summarizes recent findings on the role of WNT/β-catenin signaling in eye development. Whilst this mini-review focuses on loss-of-function and gain-of-function mutants of WNT/β-catenin signaling components, it also highlights some important aspects of β-catenin-independent WNT signaling in the eye development at later stages.

Brg1 coordinates multiple processes during retinogenesis and is a tumor suppressor in retinoblastoma

Retinal development requires precise temporal and spatial coordination of cell cycle exit, cell fate specification, cell migration and differentiation. When this process is disrupted, retinoblastoma, a developmental tumor of the retina, can form. Epigenetic modulators are central to precisely coordinating developmental events, and many epigenetic processes have been implicated in cancer. Studying epigenetic mechanisms in development is challenging because they often regulate multiple cellular processes; therefore, elucidating the primary molecular mechanisms involved can be difficult. Here we explore the role of Brg1 (Smarca4) in retinal development and retinoblastoma in mice using molecular and cellular approaches. Brg1 was found to regulate retinal size by controlling cell cycle length, cell cycle exit and cell survival during development. Brg1 was not required for cell fate specification but was required for photoreceptor differentiation and cell adhesion/polarity programs that contribute to proper retinal lamination during development. The combination of defective cell differentiation and lamination led to retinal degeneration in Brg1-deficient retinae. Despite the hypocellularity, premature cell cycle exit, increased cell death and extended cell cycle length, retinal progenitor cells persisted in Brg1-deficient retinae, making them more susceptible to retinoblastoma. ChIP-Seq analysis suggests that Brg1 might regulate gene expression through multiple mechanisms.

Summary: The SWI/SNF protein Brg1 controls cell cycle length, cell cycle exit and cell survival, and is required for cell differentiation and retinal lamination, in the developing mouse retina.

Wnt ligands from the embryonic surface ectoderm regulate 'bimetallic strip' optic cup morphogenesis in mouse

The Wnt/β-catenin response pathway is central to many developmental processes. Here, we assessed the role of Wnt signaling in early eye development using the mouse as a model system. We showed that the surface ectoderm region that includes the lens placode expressed 12 out of 19 possible Wnt ligands. When these activities were suppressed by conditional deletion of wntless (Le-cre; Wls^fl/fl) there were dramatic consequences that included a saucer-shaped optic cup, ventral coloboma, and a deficiency of periocular mesenchyme. This phenotype shared features with that produced when the Wnt/β-catenin pathway co-receptor Lrp6 is mutated or when retinoic acid (RA) signaling in the eye is compromised. Consistent with this, microarray and cell fate marker analysis identified a series of expression changes in genes known to be regulated by RA or by the Wnt/β-catenin pathway. Using pathway reporters, we showed that Wnt ligands from the surface ectoderm directly or indirectly elicit a Wnt/β-catenin response in retinal pigment epithelium (RPE) progenitors near the optic cup rim. In Le-cre; Wls^fl/fl mice, the numbers of RPE cells are reduced and this can explain, using the principle of the bimetallic strip, the curvature of the optic cup. These data thus establish a novel hypothesis to explain how differential cell numbers in a bilayered epithelium can lead to shape change.

Summary: During optic cup morphogenesis, Wnt ligands expressed in the surface ectoderm control cell proliferation in the retinal pigmented epithelium, and thus influence bending of the neural retina.

Generation of a ciliary margin-like stem cell niche from self-organizing human retinal tissue

In the developing neural retina (NR), multipotent stem cells within the ciliary margin (CM) contribute to de novo retinal tissue growth. We recently reported the ability of human embryonic stem cells (hESCs) to self-organize stratified NR using a three-dimensional culture technique. Here we report the emergence of CM-like stem cell niches within human retinal tissue. First, we developed a culture method for selective NR differentiation by timed BMP4 treatment. We then found that inhibiting GSK3 and FGFR induced the transition from NR tissue to retinal pigment epithelium (RPE), and that removing this inhibition facilitated the reversion of this RPE-like tissue back to the NR fate. This step-wise induction-reversal method generated tissue aggregates with RPE at the margin of central-peripherally polarized NR. We demonstrate that the NR-RPE boundary tissue further self-organizes a niche for CM stem cells that functions to expand the NR peripherally by de novo progenitor generation.

Epithelial morphogenesis: the mouse eye as a model system

Morphogenesis is the developmental process by which tissues and organs acquire the shape that is critical to their function. Here, we review recent advances in our understanding of the mechanisms that drive morphogenesis in the developing eye. These investigations have shown that regulation of the actin cytoskeleton is central to shaping the presumptive lens and retinal epithelia that are the major components of the eye. Regulation of the actin cytoskeleton is mediated by Rho family GTPases, by signaling pathways and indirectly, by transcription factors that govern the expression of critical genes. Changes in the actin cytoskeleton can shape cells through the generation of filopodia (that, in the eye, connect adjacent epithelia) or through apical constriction, a process that produces a wedge-shaped cell. We have also learned that one tissue can influence the shape of an adjacent one, probably by direct force transmission, in a process we term inductive morphogenesis. Though these mechanisms of morphogenesis have been identified using the eye as a model system, they are likely to apply broadly where epithelia influence the shape of organs during development.

Establishment of the neurogenic boundary of the mouse retina requires cooperation of SOX2 and WNT signaling

Background

Eye development in vertebrates relies on the critical regulation of SOX2 expression. Humans with mutations in SOX2 often suffer from eye defects including anophthalmia (no eye) and microphthalmia (small eye). In mice, deletion of Sox2 in optic cup progenitor cells results in loss of neural competence and cell fate conversion of the neural retina to a non-neurogenic fate, specifically the acquisition of fate associated with progenitors of the ciliary epithelium. This fate is also promoted with constitutive expression of stabilized β-Catenin in the optic cup, where the WNT pathway is up-regulated. We addressed whether SOX2 co-ordinates the neurogenic boundary of the retina through modulating the WNT/β-Catenin pathway by using a genetic approach in the mouse.

Results

Upon deletion of Sox2 in the optic cup, response to WNT signaling was expanded, correlating with loss of neural competence, cell fate conversion of the neural retina to ciliary epithelium primordium and, in addition, increased cell cycle time of optic cup progenitors. Removal of Ctnnb1 rescued the cell fate conversion; however, the loss of neural competence and the proliferation defect resulting from lack of SOX2 were not overcome. Lastly, central Sox2-deficient optic cup progenitor cells exhibited WNT-independent up-regulation of D-type Cyclins.

Conclusion

We propose two distinct roles for SOX2 in the developing retina. Our findings suggest that SOX2 antagonizes the WNT pathway to maintain a neurogenic fate and, in contrast, regulates cycling of optic cup progenitors in a WNT-independent manner. Given that WNT signaling acting upstream of SOX2 has been implicated in the tumorigenicity of embryonic stem cell-derived retinal progenitor cells, our results distinguish the endogenous role of WNT signaling in early optic cup patterning and support a WNT-independent role for SOX2 in maintaining retinal progenitor cell proliferation.

Electronic supplementary material

The online version of this article (doi:10.1186/1749-8104-9-27) contains supplementary material, which is available to authorized users.

β-Catenin inactivation is a pre-requisite for chick retina regeneration

In the present study we explored the role of β-catenin in mediating chick retina regeneration. The chick can regenerate its retina by activating stem/progenitor cells present in the ciliary margin (CM) of the eye or via transdifferentiation of the retinal pigmented epithelium (RPE). Both modes require fibroblast growth factor 2 (FGF2). We observed, by immunohistochemistry, dynamic changes of nuclear β-catenin in the CM and RPE after injury (retinectomy). β-catenin nuclear accumulation was transiently lost in cells of the CM in response to injury alone, while the loss of nuclear β-catenin was maintained as long as FGF2 was present. However, nuclear β-catenin positive cells remained in the RPE in response to injury and were BrdU-/p27+, suggesting that nuclear β-catenin prevents those cells from entering the cell cycle. If FGF2 is present, the RPE undergoes dedifferentiation and proliferation concomitant with loss of nuclear β-catenin. Moreover, retinectomy followed by disruption of active β-catenin by using a signaling inhibitor (XAV939) or over-expressing a dominant negative form of Lef-1 induces regeneration from both the CM and RPE in the absence of FGF2. Our results imply that β-catenin protects cells of the CM and RPE from entering the cell cycle in the developing eye, and specifically for the RPE during injury. Thus inactivation of β-catenin is a pre-requisite for chick retina regeneration.

Proliferation potential of Müller glia after retinal damage varies between mouse strains

Retinal Müller glia can serve as a source for regeneration of damaged retinal neurons in fish, birds and mammals. However, the proliferation rate of Müller glia has been reported to be low in the mammalian retina. To overcome this problem, growth factors and morphogens have been studied as potent promoters of Müller glial proliferation, but the molecular mechanisms that limit the proliferation of Müller glia in the mammalian retina remain unknown. In the present study, we found that the degree of damage-induced Müller glia proliferation varies across mouse strains. In mouse line 129×1/SvJ (129), there was a significantly larger proliferative response compared with that observed in C57BL/6 (B6) after photoreceptor cell death. Treatment with a Glycogen synthase kinase 3 (GSK3) inhibitor enhanced the proliferation of Müller glia in 129 but not in B6 mouse retinas. We therefore focused on the different gene expression patterns during retinal degeneration between B6 and 129. Expression levels of Cyclin D1 and Nestin correlated with the degree of Müller glial proliferation. A comparison of genome-wide gene expression between B6 and 129 showed that distinct sets of genes were upregulated in the retinas after damage, including immune response genes and chromatin remodeling factors.

Rax-CreERT2 knock-in mice: a tool for selective and conditional gene deletion in progenitor cells and radial glia of the retina and hypothalamus

To study gene function in neural progenitors and radial glia of the retina and hypothalamus, we developed a Rax-CreERT2 mouse line in which a tamoxifen-inducible Cre recombinase is inserted into the endogenous Rax locus. By crossing Rax-CreER(T2) with the Cre-dependent Ai9 reporter line, we demonstrate that tamoxifen-induced Cre activity recapitulates the endogenous Rax mRNA expression pattern. During embryonic development, Cre recombinase activity in Rax-CreER(T2) is confined to retinal and hypothalamic progenitor cells, as well as progenitor cells of the posterior pituitary. At postnatal time points, selective Cre recombinase activity is seen in radial glial-like cell types in these organs--specifically Müller glia and tanycytes--as well as pituicytes. We anticipate that this line will prove useful for cell lineage analysis and investigation of gene function in the developing and mature retina, hypothalamus and pituitary.

Development of the ciliary body: morphological changes in the distal portion of the optic cup in the human

This study seeks to determine the main events that occur in the development of the ciliary body (CB) in the 5-14th week of development. The CB develops from the distal portion of the optic cup (OC) and the neighboring mesenchyme. During the 5th week of development, 4 zones were observed in the distal portion of the OC: in zone 1, the epithelia of the outer and inner layers of the OC came into contact. This contact coincided with the appearance of mainly apical granule pigments. This zone corresponded to the anlage of the epithelial layers of the CB. In zone 2, the cells surrounded the marginal sinus and contained scarce pigment granules and nuclei in the basal position. This zone corresponded to the anlage of the iris. Zone 3 was triangular in shape and its vertex ran towards the marginal sinus and corresponded to common cell progenitors. Zone 4 corresponded to the retinal pigment epithelium anlage and the neural retina anlage. We determined the onset of the stroma and the ciliary muscle anlage at the end of the 7th week. In the 13-14th week, we observed the anlage of the orbicularis ciliaris (pars plana of the CB) and corona ciliaris (pars plicata of the CB), in addition to the anlage of the ciliary muscle. Our study, therefore, establishes a precise timetable of the development of the CB.

Notch2 regulates BMP signaling and epithelial morphogenesis in the ciliary body of the mouse eye

The ciliary body (CB) of the mammalian eye is responsible for secreting aqueous humor to maintain intraocular pressure, which is elevated in the eyes of glaucoma patients. It contains a folded two-layered epithelial structure comprising the nonpigmented inner ciliary epithelium (ICE), the pigmented outer ciliary epithelium (OCE), and the underlying stroma. Although the CB has an important function in the eye, its morphogenesis remains poorly studied. In this study, we show that conditional inactivation of the Jagged 1 (Jag1)-Notch2 signaling pathway in the developing CB abolishes its morphogenesis. Notch2 is expressed in the OCE of the CB, whereas Jag1 is expressed in the ICE. Conditional inactivation of Jag1 in the ICE or Notch2 in the OCE disrupts CB morphogenesis, but neither affects the specification of the CB region. Notch2 signaling in the OCE is required for promoting cell proliferation and maintaining bone morphogenetic protein (BMP) signaling, both of which have been suggested to be important for CB morphogenesis. Although Notch and BMP signaling pathways are known to cross-talk via the interaction between their downstream transcriptional factors, this study suggests that Notch2 maintains BMP signaling in the OCE possibly by repressing expression of secreted BMP inhibitors. Based on our findings, we propose that Jag1-Notch2 signaling controls CB morphogenesis at least in part by regulating cell proliferation and BMP signaling.

Foxg1 is required to limit the formation of ciliary margin tissue and Wnt/β-catenin signalling in the developing nasal retina of the mouse

The ciliary margin (CM) develops in the peripheral retina and gives rise to the iris and the ciliary body. The Wnt/β-catenin signalling pathway has been implicated in ciliary margin development. Here, we tested the hypothesis that in the developing mouse retina Foxg1 is responsible for suppressing the Wnt/β-catenin pathway and restricting CM development. We showed that there is excess CM tissue in Foxg1^−/− null embryos and this expansion is more pronounced in the nasal retina where Foxg1 normally shows its highest expression levels. Results on expression of a reporter allele for Wnt/β-catenin signalling and of Lef1, a target of Wnt/β-catenin signalling, displayed significant upregulation of this pathway in Foxg1^−/− nulls at embryonic days 12.5 and 14.5. Interestingly, this upregulation was observed specifically in the nasal retina, where normally very few Wnt-responsive cells are observed. These results indicate a suppressive role of Foxg1 on this signalling pathway. Our results reveal a new role of Foxg1 in limiting CM development in the nasal peripheral retina and add a new molecular player in the developmental network involved in CM specification.

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Foxg1 is expressed in a nasal-high to temporal-low gradient in developing retina.

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Ciliary margin expansion is observed nasally in the Foxg1^−/− mutant retina.

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Wnt/β-catenin signalling is upregulated in the Foxg1^−/− peripheral retina nasally.

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A new role of Foxg1 in controlling ciliary margin development is proposed.

Eye development and retinogenesis

Three embryonic tissue sources-the neural ectoderm, the surface ectoderm, and the periocular mesenchyme-contribute to the formation of the mammalian eye. For this reason, the developing eye has presented an invaluable system for studying the interactions among cells and, more recently, genes, in specifying cell fate. This article describes how the eye primordium is specified in the anterior neural plate by four eye field transcription factors and how the optic vesicle becomes regionalized into three distinct tissue types. Specific attention is given to how cross talk between the optic vesicle and surface ectoderm contributes to lens and optic cup formation. This article also describes how signaling networks and cell movements set up axes in the optic cup and establish the multiple cell fates important for vision. How multipotent retinal progenitor cells give rise to the six neuronal and one glial cell type in the mature retina is also explained. Finally, the history and progress of cellular therapeutics for the treatment of degenerative eye disease is outlined. Throughout this article, special attention is given to how disruption of gene function causes ocular malformation in humans. Indeed, the accessibility of the eye has contributed much to our understanding of the basic processes involved in mammalian development.

Gene networks: dissecting pathways in retinal development and disease

During retinal neurogenesis, diverse cellular subtypes originate from multipotent neural progenitors in a spatiotemporal order leading to a highly specialized laminar structure combined with a distinct mosaic architecture. This is driven by the combinatorial action of transcription factors and signaling molecules which specify cell fate and differentiation. The emerging approach of gene network analysis has allowed a better understanding of the functional relationships between genes expressed in the developing retina. For instance, these gene networks have identified transcriptional hubs that have revealed potential targets and pathways for the development of therapeutic options for retinal diseases. Much of the current knowledge has been informed by targeted gene deletion experiments and gain-of-functional analysis. In this review we will provide an update on retinal development gene networks and address the wider implications for future disease therapeutics.

Single-cell profiling of developing and mature retinal neurons

Highly specialized, but exceedingly small populations of cells play important roles in many tissues. The identification of cell-type specific markers and gene expression programs for extremely rare cell subsets has been a challenge using standard whole-tissue approaches. Gene expression profiling of individual cells allows for unprecedented access to cell types that comprise only a small percentage of the total tissue(1-7). In addition, this technique can be used to examine the gene expression programs that are transiently expressed in small numbers of cells during dynamic developmental transitions(8). This issue of cellular diversity arises repeatedly in the central nervous system (CNS) where neuronal connections can occur between quite diverse cells(9). The exact number of distinct cell types is not precisely known, but it has been estimated that there may be as many as 1000 different types in the cortex itself(10). The function(s) of complex neural circuits may rely on some of the rare neuronal types and the genes they express. By identifying new markers and helping to molecularly classify different neurons, the single-cell approach is particularly useful in the analysis of cell types in the nervous system. It may also help to elucidate mechanisms of neural development by identifying differentially expressed genes and gene pathways during early stages of neuronal progenitor development. As a simple, easily accessed tissue with considerable neuronal diversity, the vertebrate retina is an excellent model system for studying the processes of cellular development, neuronal differentiation and neuronal diversification. However, as in other parts of the CNS, this cellular diversity can present a problem for determining the genetic pathways that drive retinal progenitors to adopt a specific cell fate, especially given that rod photoreceptors make up the majority of the total retinal cell population(11). Here we report a method for the identification of the transcripts expressed in single retinal cells (Figure 1). The single-cell profiling technique allows for the assessment of the amount of heterogeneity present within different cellular populations of the retina(2,4,5,12). In addition, this method has revealed a host of new candidate genes that may play role(s) in the cell fate decision-making processes that occur in subsets of retinal progenitor cells(8). With some simple adjustments to the protocol, this technique can be utilized for many different tissues and cell types.

Identification of Wnt/β-catenin modulated genes in the developing retina

PURPOSE

During mammalian eye development, the restriction of Wnt/β-catenin signaling at the junction of the neural retina and the retinal pigment epithelium in the peripheral eyecup is required for the development of the ciliary margin, a non-neural region of the eyecup that is the precursor of the ciliary body and iris of the adult eye.

METHODS

To identify genes that are modulated by β-catenin activity in the embryonic retina, we performed gene expression profiling in Li(+)-treated retinal explants, a pharmacological model of β-catenin activation. The Li(+)-modulated gene data set was searched for β-catenin/T-cell specific transcription factor binding sites.

RESULTS

Functional annotations of this data set revealed significant enrichments for genes involved in chromatin organization, neurogenesis, and cell motion/migration. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis confirmed the modulation of 12 genes in Li(+)-treated explants and retinas of mice with Cre-mediated induction of constitutively active β-catenin (β-cat(act)). In situ hybridization revealed β-catenin-specific upregulation of cyclin-dependent kinase inhibitor 1A (P21) [Cdkn1a] and tumor necrosis factor receptor superfamily, member 19 (Tnfrsf19) in the developing retina consistent with the antineurogenic and proliferation changes associated with ectopic Wnt/β-catenin signaling in the eyecup.

CONCLUSIONS

This data set of Li(+)-modulated genes provides a valuable resource for characterizing the Wnt/ β-catenin regulated gene network in eyecup patterning.

Retinal and anterior eye compartments derive from a common progenitor pool in the avian optic cup

PURPOSE

The optic cup is created through invagination of the optic vesicle. The morphogenetic rearrangement creates a double-layered cup, with a hinge (the Optic Cup Lip) where the epithelium bends back upon itself. Shortly after the optic cup forms, it is thought to be sub-divided into separate lineages: i) pigmented epithelium in the outer layer; ii) presumptive iris and ciliary body at the most anterior aspect of the inner layer; and iii) presumptive neural retina in the remainder of the inner layer. We test the native developmental potential of the anterior cup to determine if it normally contributes to the retina.

METHODS

Vital dye and green fluorescent protein (GFP) expressing replication-incompetent retroviral vectors were used to label cells in the nascent optic cup and follow their direct progeny throughout development. Label was applied to either the optic cup lip (n=40), or to the domain just posterior to the lip (n=20). Retroviral labeling is a permanent lineage marker and enabled the analysis of advanced stages of development.

RESULTS

Labeling within the optic cup gave rise to labeled progeny in the posterior optic cup that differentiated as neural retina (20 of 20). In contrast, labeling cells in the optic cup lip gave rise to progeny of labeled cells arrayed in a linear progression, from the lip into the neural retina (36 of 40). Label was retained in cells at the optic cup lip, regardless of age at examination. In older embryos, labeled progeny delaminated from the optic cup lip to differentiate as muscle of the pupillary margin.

CONCLUSIONS

The data show that the cells at the optic cup lip are a common progenitor population for pigmented epithelium, anterior eye tissues (ciliary body, iris, and pupillary muscle) and retinal neurons. The findings are supportive of an interpretation where the optic cup lip is a specialized niche containing a multipotent progenitor population.

Secreted frizzled-related proteins are required for Wnt/β-catenin signalling activation in the vertebrate optic cup

Secreted frizzled-related proteins (Sfrps) are considered Wnt signalling antagonists but recent studies have shown that specific family members enhance Wnt diffusion and thus positively modulate Wnt signalling. Whether this is a general and physiological property of all Sfrps remains unexplored. It is equally unclear whether disruption of Sfrp expression interferes with developmental events mediated by Wnt signalling activation. Here, we have addressed these questions by investigating the functional consequences of Sfrp disruption in the canonical Wnt signalling-dependent specification of the mouse optic cup periphery. We show that compound genetic inactivation of Sfrp1 and Sfrp2 prevents Wnt/β-catenin signalling activation in this structure, which fails to be specified and acquires neural retina characteristics. Consistent with a positive role of Sfrps in signalling activation, Wnt spreading is impaired in the retina of Sfrp1(-/-);Sfrp2(-/-) mice. Conversely, forced expression of Sfrp1 in the wing imaginal disc of Drosophila, the only species in which the endogenous Wnt distribution can be detected, flattens the Wg gradient, suppresses the expression of high-Wg target genes but expands those typically activated by low Wg concentrations. Collectively, these data demonstrate that, in vivo, the levels of Wnt signalling activation strongly depend on the tissue distribution of Sfrps, which should be viewed as multifunctional regulators of Wnt signalling.

Tsukushi functions as a Wnt signaling inhibitor by competing with Wnt2b for binding to transmembrane protein Frizzled4

The Wnt signaling pathway is essential for the development of diverse tissues during embryogenesis. Signal transduction is activated by the binding of Wnt proteins to the type I receptor low-density lipoprotein receptor-related protein 5/6 and the seven-pass transmembrane protein Frizzled (Fzd), which contains a Wnt-binding site in the form of a cysteine-rich domain. Known extracellular antagonists of the Wnt signaling pathway can be subdivided into two broad classes depending on whether they bind primarily to Wnt or to low-density lipoprotein receptor-related protein 5/6. We show that the secreted protein Tsukushi (TSK) functions as a Wnt signaling inhibitor by binding directly to the cysteine-rich domain of Fzd4 with an affinity of 2.3 × 10(-10) M and competing with Wnt2b. In the developing chick eye, TSK is expressed in the ciliary/iris epithelium, whereas Wnt2b is expressed in the adjacent anterior rim of the optic vesicle, where it controls the differentiation of peripheral eye structures, such as the ciliary body and iris. TSK overexpression effectively antagonizes Wnt2b signaling in chicken embryonic retinal cells both in vivo and in vitro and represses Wnt-dependent specification of peripheral eye fates. Conversely, targeted inactivation of the TSK gene in mice causes expansion of the ciliary body and up-regulation of Wnt2b and Fzd4 expression in the developing peripheral eye. Thus, we uncover a crucial role for TSK as a Wnt signaling inhibitor that regulates peripheral eye formation.

Mouse retinal development: a dark horse model for systems biology research

The developing retina is an excellent model to study cellular fate determination and differentiation in the context of a complex tissue. Over the last decade, many basic principles and key genes that underlie these processes have been experimentally identified. In this review, we construct network models to summarize known gene interactions that underlie determination and fundamentally affect differentiation of each retinal cell type. These networks can act as a scaffold to assemble subsequent discoveries. In addition, these summary networks provide a rational segue to systems biology approaches necessary to understand the many events leading to appropriate cellular determination and differentiation in the developing retina and other complex tissues.

Combinatorial regulation of optic cup progenitor cell fate by SOX2 and PAX6

In humans, haploinsufficiency of either SOX2 or PAX6 is associated with microphthalmia, anophthalmia or aniridia. In this study, through the genetic spatiotemporal specific ablation of SOX2 on both wild-type and Pax6-haploinsufficent backgrounds in the mouse, we have uncovered a transcriptionally distinct and developmentally transient stage of eye development. We show that genetic ablation of SOX2 in the optic cup results in complete loss of neural competence and eventual cell fate conversion to non-neurogenic ciliary epithelium. This cell fate conversion is associated with a striking increase in PAX6, and genetically ablating SOX2 on a Pax6-haploinsufficient background partially rescues the Sox2-mutant phenotype. Collectively, these results demonstrate that precise regulation of the ratio of SOX2 to PAX6 is necessary to ensure accurate progenitor cell specification, and place SOX2 as a decisive factor of neural competence in the retina.