Negative regulation of Vsx1 by its paralog Chx10/Vsx2 is conserved in the vertebrate retina

Microphthalmia and Disrupted Retinal Development Due to a LacZ Knock-in/Knock-Out Allele at the Vsx2 Locus

Purpose

Visual System Homeobox 2 (Vsx2) is a transcription factor expressed in the developing retina that regulates tissue identity, growth, and fate determination. Several mutations in the Vsx2 gene exist in mice, including a spontaneous nonsense mutation and two targeted missense mutations originally identified in humans. Here, we expand the genetic repertoire to include a LacZ reporter allele (Vsx2LacZ ) designed to express beta-Galactosidase (bGal) and simultaneously disrupt Vsx2 function (knock-in/knock-out).

Methods

We generated a Vsx2LacZ reporter allele with an in-frame fusion to the Vsx2 coding sequence immediately following exon 2. Germline transmission was assessed with genomic DNA PCR and Western blot analysis was used to describe VSX2 expression from the mutant allele (LacZ). Eye size quantification and immunohistology were used to describe the embryonic and postnatal retinal phenotypes of LacZ homozygous and heterozygous mice. The contribution of Mitf to LacZ mutant microphthalmia was probed with the semi-dominant negative Mitfmi allele.

Results

The retinal expression pattern of bGal is concordant with VSX2, and the mutant allele is recessive. Vsx2LacZ homozygous mice have congenital bilateral microphthalmia accompanied by defects in retinal development including ectopic expression of non-retinal genes, reduced proliferation, delayed neurogenesis, aberrant tissue morphology, and an absence of bipolar interneurons - all hallmarks of Vsx2 loss-of-function. The Mitfmi allele reduced the severity of microphthalmia caused by the Vsx2LacZ allele. Unexpectedly, the mutant VSX2 protein is stably expressed, and there are subtle differences in eye size and early retinal neurogenesis when compared to the null mutant, ocular retardation J.

Conclusion

The perdurance of the mutant VSX2 protein combined with subtle deviations from the null phenotype leaves open the possibility that Vsx2LacZ allele is not a complete knock-out. The Vsx2LacZ allele exhibits loss-of-function characteristics and adds to the genetic toolkit for understanding Vsx2 function.

Cellular and Molecular Determinants of Retinal Cell Fate

The vertebrate retina is regarded as a simple part of the central nervous system (CNS) and thus amenable to investigations of the determinants of cell fate. Its five neuronal cell classes and one glial cell class all derive from a common pool of progenitors. Here we review how each cell class is generated. Retinal progenitors progress through different competence states, in each of which they generate only a small repertoire of cell classes. The intrinsic state of the progenitor is determined by the complement of transcription factors it expresses. Thus, although progenitors are multipotent, there is a bias in the types of fates they generate during any particular time window. Overlying these competence states are stochastic mechanisms that influence fate decisions. These mechanisms are determined by a weighted set of probabilities based on the abundance of a cell class in the retina. Deterministic mechanisms also operate, especially late in development, when preprogrammed progenitors solely generate specific fates.

Functional analysis of the Vsx2 super-enhancer uncovers distinct cis-regulatory circuits controlling Vsx2 expression during retinogenesis

Vsx2 is a transcription factor essential for retinal proliferation and bipolar cell differentiation, but the molecular mechanisms underlying its developmental roles are unclear. Here, we have profiled VSX2 genomic occupancy during mouse retinogenesis, revealing extensive retinal genetic programs associated with VSX2 during development. VSX2 binds and transactivates its enhancer in association with the transcription factor PAX6. Mice harboring deletions in the Vsx2 regulatory landscape exhibit specific abnormalities in retinal proliferation and in bipolar cell differentiation. In one of those deletions, a complete loss of bipolar cells is associated with a bias towards photoreceptor production. VSX2 occupies cis-regulatory elements nearby genes associated with photoreceptor differentiation and homeostasis in the adult mouse and human retina, including a conserved region nearby Prdm1, a factor implicated in the specification of rod photoreceptors and suppression of bipolar cell fate. VSX2 interacts with the transcription factor OTX2 and can act to suppress OTX2-dependent enhancer transactivation of the Prdm1 enhancer. Taken together, our analyses indicate that Vsx2 expression can be temporally and spatially uncoupled at the enhancer level, and they illuminate important mechanistic insights into how VSX2 is engaged with gene regulatory networks that are essential for retinal proliferation and cell fate acquisition.

Self-Organization of the Retina during Eye Development, Retinal Regeneration In Vivo, and in Retinal 3D Organoids In Vitro

Self-organization is a process that ensures histogenesis of the eye retina. This highly intricate phenomenon is not sufficiently studied due to its biological complexity and genetic heterogeneity. The review aims to summarize the existing central theories and ideas for a better understanding of retinal self-organization, as well as to address various practical problems of retinal biomedicine. The phenomenon of self-organization is discussed in the spatiotemporal context and illustrated by key findings during vertebrate retina development in vivo and retinal regeneration in amphibians in situ. Described also are histotypic 3D structures obtained from the disaggregated retinal progenitor cells of birds and retinal 3D organoids derived from the mouse and human pluripotent stem cells. The review highlights integral parts of retinal development in these conditions. On the cellular level, these include competence, differentiation, proliferation, apoptosis, cooperative movements, and migration. On the physical level, the focus is on the mechanical properties of cell- and cell layer-derived forces and on the molecular level on factors responsible for gene regulation, such as transcription factors, signaling molecules, and epigenetic changes. Finally, the self-organization phenomenon is discussed as a basis for the production of retinal organoids, a promising model for a wide range of basic scientific and medical applications.

Somatostatin signalling promotes the differentiation of rod photoreceptors in human pluripotent stem cell-derived retinal organoid

Objectives

Stem cell‐derived photoreceptor replacement therapy is a promising strategy for the treatment of retinal degenerative disease. The development of 3D retinal organoids has permitted the production of photoreceptors. However, there is no strategy to enrich a specific photoreceptor subtype due to inadequate knowledge of the molecular mechanism underlying the photoreceptor fate determination. Hence, our aim is to explore the uncharacterized function of somatostatin signalling in human pluripotent stem cell‐derived photoreceptor differentiation.

Materials and Methods

3D retinal organoids were achieved from human embryonic stem cell. The published single‐cell RNA‐sequencing datasets of human retinal development were utilized to further investigate the transcriptional regulation of photoreceptor differentiation. The assays of immunofluorescence staining, lentivirus transfection, real‐time quantitative polymerase chain reaction and western blotting were performed.

Results

We identified that the somatostatin receptor 2 (SSTR2)‐mediated signalling was essential for rod photoreceptor differentiation at the precursor stage. The addition of the cognate ligand somatostatin in human 3D retinal organoids promoted rod photoreceptor differentiation and inhibited cone photoreceptor production. Furthermore, we found that the genesis of rod photoreceptors was modulated by endogenous somatostatin specifically secreted by developing retinal ganglion cells.

Conclusions

Our study identified SSTR2 signalling as a novel extrinsic regulator for rod photoreceptor fate determination in photoreceptor precursors, which expands the repertoire of functional signalling pathways in photoreceptor development and sheds light on the optimization of the photoreceptor enrichment strategy.

Development and diversification of bipolar interneurons in the mammalian retina

The bipolar interneurons of the mammalian retina have evolved as a diverse set of cells with distinct subtype characteristics, which reflect specialized contributions to visual circuitry. Fifteen subtypes of bipolar interneurons have been identified in the mouse retina, each with characteristic gene expression, morphology, and light responses. This review provides an overview of the developmental events that underlie the generation of the diverse bipolar cell class, summarizing the current knowledge of genetic programs that establish and maintain bipolar subtype fates, as well as the events that shape the final distribution of bipolar subtypes. With much left to be discovered, bipolar interneurons present an ideal model system for studying the interplay between cell-autonomous and non-cell-autonomous mechanisms that influence neuronal subtype development within the central nervous system.

Generation and characterization of a tamoxifen-inducible Vsx1-CreERT2 line to target V2 interneurons in the mouse developing spinal cord

In the spinal cord, ventral interneurons regulate the activity of motor neurons, thereby controlling motor activities including locomotion. Interneurons arise during embryonic development from distinct progenitor domains orderly distributed along the dorso-ventral axis of the neural tube. The p2 progenitor domain generates at least five V2 interneuron populations. However, identification and characterization of all V2 populations remain currently incomplete and the mechanisms that control their development remain only partly understood. In this study, we report the generation of a Vsx1-CreER^T2 BAC transgenic mouse line that drives CreER^T2 recombinase expression mimicking endogenous Vsx1 expression pattern in the developing spinal cord. We showed that the Vsx1-CreER^T2 transgene can mediate recombination in V2 precursors with a high efficacy and specificity. Lineage tracing demonstrated that all the V2 interneurons in the mouse developing spinal cord derive from cells expressing Vsx1. Finally, we confirmed that V2 precursors generate additional V2 populations that are not characterized yet. Thus, the Vsx1-CreER^T2 line described here is a useful genetic tool for lineage tracing and for functional studies of the mouse spinal V2 interneurons.

Functional and Evolutionary Diversification of Otx2 and Crx in Vertebrate Retinal Photoreceptor and Bipolar Cell Development

Otx family homeoproteins Otx2 and Crx are expressed in photoreceptor precursor cells and bind to the common DNA-binding consensus sequence, but these two proteins have distinct functions in retinal development. To examine the functional substitutability of Otx2 and Crx, we generate knockin mouse lines in which Crx is replaced by Otx2 and vice versa. We find that Otx2 and Crx cannot be substituted in photoreceptor development. Subsequently, we investigate the function of Otx2 in photoreceptor and bipolar cell development. High Otx2 levels induce photoreceptor cell fate but not bipolar cell fate, whereas reduced Otx2 expression impairs bipolar cell maturation and survival. Furthermore, we identify Otx2 and Crx in the lamprey genome by using synteny analysis, suggesting that the last common ancestor of vertebrates possesses both Otx2 and Crx. We find that the retinal Otx2 expression pattern is different between lampreys and mice, suggesting that neofunctionalization of Otx2 occurred in the jawed vertebrate lineage.

Vsx1 and Chx10 paralogs sequentially secure V2 interneuron identity during spinal cord development

Paralog factors are usually described as consolidating biological systems by displaying redundant functionality in the same cells. Here, we report that paralogs can also cooperate in distinct cell populations at successive stages of differentiation. In mouse embryonic spinal cord, motor neurons and V2 interneurons differentiate from adjacent progenitor domains that share identical developmental determinants. Therefore, additional strategies secure respective cell fate. In particular, Hb9 promotes motor neuron identity while inhibiting V2 differentiation, whereas Chx10 stimulates V2a differentiation while repressing motor neuron fate. However, Chx10 is not present at the onset of V2 differentiation and in other V2 populations. In the present study, we show that Vsx1, the single paralog of Chx10, which is produced earlier than Chx10 in V2 precursors, can inhibit motor neuron differentiation and promote V2 interneuron production. However, the single absence of Vsx1 does not impact on V2 fate consolidation, suggesting that lack of Vsx1 may be compensated by other factors. Nevertheless, Vsx1 cooperates with Chx10 to prevent motor neuron differentiation in early V2 precursors although these two paralog factors are not produced in the same cells. Hence, this study uncovers an original situation, namely labor division, wherein paralog genes cooperate at successive steps of neuronal development.

The Use of Induced Pluripotent Stem Cells as a Model for Developmental Eye Disorders

Approximately one-third of childhood blindness is attributed to developmental eye disorders, of which 80% have a genetic cause. Eye morphogenesis is tightly regulated by a highly conserved network of transcription factors when disrupted by genetic mutations can result in severe ocular malformation. Human-induced pluripotent stem cells (hiPSCs) are an attractive tool to study early eye development as they are more physiologically relevant than animal models, can be patient-specific and their use does not elicit the ethical concerns associated with human embryonic stem cells. The generation of self-organizing hiPSC-derived optic cups is a major advancement to understanding mechanisms of ocular development and disease. Their development in vitro has been found to mirror that of the human eye and these early organoids have been used to effectively model microphthalmia caused by a VSX2 variant. hiPSC-derived optic cups, retina, and cornea organoids are powerful tools for future modeling of disease phenotypes and will enable a greater understanding of the pathophysiology of many other developmental eye disorders. These models will also provide an effective platform for identifying molecular therapeutic targets and for future clinical applications.

Simultaneous deletion of Prdm1 and Vsx2 enhancers in the retina alters photoreceptor and bipolar cell fate specification, yet differs from deleting both genes

The transcription factor OTX2 is required for photoreceptor and bipolar cell formation in the retina. It directly activates the transcription factors Prdm1 and Vsx2 through cell type-specific enhancers. PRDM1 and VSX2 work in opposition, such that PRDM1 promotes photoreceptor fate and VSX2 bipolar cell fate. To determine how OTX2+ cell fates are regulated in mice, we deleted Prdm1 and Vsx2 or their cell type-specific enhancers simultaneously using a CRISPR/Cas9 in vivo retina electroporation strategy. Double gene or enhancer targeting effectively removed PRDM1 and VSX2 protein expression. However, double enhancer targeting favored bipolar fate outcomes, whereas double gene targeting favored photoreceptor fate. Both conditions generated excess amacrine cells. Combined, these fate changes suggest that photoreceptors are a default fate outcome in OTX2+ cells and that VSX2 must be present in a narrow temporal window to drive bipolar cell formation. Prdm1 and Vsx2 also appear to redundantly restrict the competence of OTX2+ cells, preventing amacrine cell formation. By taking a combinatorial deletion approach of both coding sequences and enhancers, our work provides new insights into the complex regulatory mechanisms that control cell fate choice.

Zygotic Vsx1 Plays a Key Role in Defining V2a Interneuron Sub-Lineage by Directly Repressing tal1 Transcription in Zebrafish

In the spinal cord, excitatory V2a and inhibitory V2b interneurons are produced together by the final division of common P2 progenitors. During V2a and V2b diversification, Tal1 is necessary and sufficient to promote V2b differentiation and Vsx2 suppresses the expression of motor neuron genes to consolidate V2a interneuron identity. The expression program of Tal1 is triggered by a Foxn4-driven regulatory network in the common P2 progenitors. Why the expression of Tal1 is inhibited in V2a interneurons at the onset of V2a and V2b sub-lineage diversification remains unclear. Since transcription repressor Vsx1 is expressed in the P2 progenitors and newborn V2a cells in zebrafish, we investigated the role of Vsx1 in V2a fate specification during V2a and V2b interneuron diversification in this species by loss and gain-of-function experiments. In vsx1 knockdown embryos or knockout Go chimeric embryos, tal1 was ectopically expressed in the presumptive V2a cells, while the generation of V2a interneurons was significantly suppressed. By contrast, in vsx1 overexpression embryos, normal expression of tal1 in the presumptive V2b cells was suppressed, while the generation of V2a interneuron was expanded. Chromatin immunoprecipitation and electrophoretic mobility shift assays in combination with core consensus sequence mutation analysis further revealed that Vsx1 can directly bind to tal1 promoter and repress tal1 transcription. These results indicate that Vsx1 can directly repress tal1 transcription and plays an essential role in defining V2a interneuron sub-lineage during V2a and V2b sub-lineage diversification in zebrafish.

Meta-analysis of 542,934 subjects of European ancestry identifies new genes and mechanisms predisposing to refractive error and myopia

Refractive errors, in particular myopia, are a leading cause of morbidity and disability worldwide. Genetic investigation can improve understanding of the molecular mechanisms that underlie abnormal eye development and impaired vision. We conducted a meta-analysis of genome-wide association studies (GWAS) that involved 542,934 European participants and identified 336 novel genetic loci associated with refractive error. Collectively, all associated genetic variants explain 18.4% of heritability and improve the accuracy of myopia prediction (area under the curve (AUC) = 0.75). Our results suggest that refractive error is genetically heterogeneous, driven by genes that participate in the development of every anatomical component of the eye. In addition, our analyses suggest that genetic factors controlling circadian rhythm and pigmentation are also involved in the development of myopia and refractive error. These results may enable the prediction of refractive error and the development of personalized myopia prevention strategies in the future.

Inherited Eye Diseases with Retinal Manifestations through the Eyes of Homeobox Genes

Retinal development is under the coordinated control of overlapping networks of signaling pathways and transcription factors. The paper was conceived as a review of the data and ideas that have been formed to date on homeobox genes mutations that lead to the disruption of eye organogenesis and result in inherited eye/retinal diseases. Many of these diseases are part of the same clinical spectrum and have high genetic heterogeneity with already identified associated genes. We summarize the known key regulators of eye development, with a focus on the homeobox genes associated with monogenic eye diseases showing retinal manifestations. Recent advances in the field of genetics and high-throughput next-generation sequencing technologies, including single-cell transcriptome analysis have allowed for deepening of knowledge of the genetic basis of inherited retinal diseases (IRDs), as well as improve their diagnostics. We highlight some promising avenues of research involving molecular-genetic and cell-technology approaches that can be effective for IRDs therapy. The most promising neuroprotective strategies are aimed at mobilizing the endogenous cellular reserve of the retina.

Cis-regulatory basis of sister cell type divergence in the vertebrate retina

Multicellular organisms evolved via repeated functional divergence of transcriptionally related sister cell types, but the mechanisms underlying sister cell type divergence are not well understood. Here, we study a canonical pair of sister cell types, retinal photoreceptors and bipolar cells, to identify the key cis-regulatory features that distinguish them. By comparing open chromatin maps and transcriptomic profiles, we found that while photoreceptor and bipolar cells have divergent transcriptomes, they share remarkably similar cis-regulatory grammars, marked by enrichment of K50 homeodomain binding sites. However, cell class-specific enhancers are distinguished by enrichment of E-box motifs in bipolar cells, and Q50 homeodomain motifs in photoreceptors. We show that converting K50 motifs to Q50 motifs represses reporter expression in bipolar cells, while photoreceptor expression is maintained. These findings suggest that partitioning of Q50 motifs within cell type-specific cis-regulatory elements was a critical step in the evolutionary divergence of the bipolar transcriptome from that of photoreceptors.

Requirements for Neurogenin2 during mouse postnatal retinal neurogenesis

During embryonic retinal development, the bHLH factor Neurog2 regulates the temporal progression of neurogenesis, but no role has been assigned for this gene in the postnatal retina. Using Neurog2 conditional mutants, we found that Neurog2 is necessary for the development of an early, embryonic cohort of rod photoreceptors, but also required by both a subset of cone bipolar subtypes, and rod bipolars. Using transcriptomics, we identified a subset of downregulated genes in P2 Neurog2 mutants, which act during rod differentiation, outer segment morphogenesis or visual processing. We also uncovered defects in neuronal cell culling, which suggests that the rod and bipolar cell phenotypes may arise via more complex mechanisms rather than a simple cell fate shift. However, given an overall phenotypic resemblance between Neurog2 and Blimp1 mutants, we explored the relationship between these two factors. We found that Blimp1 is downregulated between E12-birth in Neurog2 mutants, which probably reflects a dependence on Neurog2 in embryonic progenitor cells. Overall, we conclude that the Neurog2 gene is expressed and active prior to birth, but also exerts an influence on postnatal retinal neuron differentiation.

Combinatorial regulation of a Blimp1 (Prdm1) enhancer in the mouse retina

The mouse retina comprises seven major cell types that exist in differing proportions. They are generated from multipotent progenitors in a stochastic manner, such that the relative frequency of any given type generated changes over time. The mechanisms determining the proportions of each cell type are only partially understood. Photoreceptors and bipolar interneurons are derived from cells that express Otx2. Within this population, Blimp1 (Prdm1) helps set the balance between photoreceptors and bipolar cells by suppressing bipolar identity in most of the cells. How only a subset of these Otx2+ cells decides to upregulate Blimp1 and adopt photoreceptor fate is unknown. To understand this, we investigated how Blimp1 transcription is regulated. We identified several potential Blimp1 retinal enhancer elements using DNase hypersensitivity sequencing. Only one of the elements recapitulated Blimp1 spatial and temporal expression in cultured explant assays and within the retinas of transgenic mice. Mutagenesis of this retinal Blimp1 enhancer element revealed four discrete sequences that were each required for its activity. These included highly conserved Otx2 and ROR (retinoic acid receptor related orphan receptor) binding sites. The other required sequences do not appear to be controlled by Otx2 or ROR factors, increasing the complexity of the Blimp1 gene regulatory network. Our results show that the intersection of three or more transcription factors is required to correctly regulate the spatial and temporal features of Blimp1 enhancer expression. This explains how Blimp1 expression can diverge from Otx2 and set the balance between photoreceptor and bipolar fates.

Gsg1, Trnp1, and Tmem215 Mark Subpopulations of Bipolar Interneurons in the Mouse Retina

Purpose

How retinal bipolar cell interneurons are specified and assigned to specialized subtypes is only partially understood. In part, this is due to a lack of early pan- and subtype-specific bipolar cell markers. To discover these factors, we identified genes that were upregulated in Blimp1 (Prdm1) mutant retinas, which exhibit precocious bipolar cell development.

Methods

Postnatal day (P)2 retinas from Blimp1 conditional knock-out (CKO) mice and controls were processed for RNA sequencing. Genes that increased at least 45% and were statistically different between conditions were considered candidate bipolar-specific factors. Candidates were further evaluated by RT-PCR, in situ hybridization, and immunohistochemistry. Knock-in Tmem215-LacZ mice were used to better trace retinal expression.

Results

A comparison between Blimp1 CKO and control RNA-seq datasets revealed approximately 40 significantly upregulated genes. We characterized the expression of three genes that have no known function in the retina, Gsg1 (germ cell associated gene), Trnp1 (TMF-regulated nuclear protein), and Tmem215 (a predicted transmembrane protein). Germ cell associated gene appeared restricted to a small subset of cone bipolars while Trnp1 was seen in all ON type bipolar cells. Using Tmem215-LacZ heterozygous knock-in mice, we observed that β-galactosidase expression started early in bipolar cell development. In adults, Tmem215 was expressed by a subset of ON and OFF cone bipolar cells.

Conclusions

We have identified Gsg1, Tmem215, and Trnp1 as novel bipolar subtype-specific genes. The spatial and temporal pattern of their expression is consistent with a role in controlling bipolar subtype fate choice, differentiation, or physiology.

Vsx1 Transiently Defines an Early Intermediate V2 Interneuron Precursor Compartment in the Mouse Developing Spinal Cord

Spinal ventral interneurons regulate the activity of motor neurons, thereby controlling motor activities. Interneurons arise during embryonic development from distinct progenitor domains distributed orderly along the dorso-ventral axis of the neural tube. A single ventral progenitor population named p2 generates at least five V2 interneuron subsets. Whether the diversification of V2 precursors into multiple subsets occurs within the p2 progenitor domain or involves a later compartment of early-born V2 interneurons remains unsolved. Here, we provide evidence that the p2 domain produces an intermediate V2 precursor compartment characterized by the transient expression of the transcriptional repressor Vsx1. These cells display an original repertoire of cellular markers distinct from that of any V2 interneuron population. They have exited the cell cycle but have not initiated neuronal differentiation. They coexpress Vsx1 and Foxn4, suggesting that they can generate the known V2 interneuron populations as well as possible additional V2 subsets. Unlike V2 interneurons, the generation of Vsx1-positive precursors does not depend on the Notch signaling pathway but expression of Vsx1 in these cells requires Pax6. Hence, the p2 progenitor domain generates an intermediate V2 precursor compartment, characterized by the presence of the transcriptional repressor Vsx1, that contributes to V2 interneuron development.

Regulation of WNT Signaling by VSX2 During Optic Vesicle Patterning in Human Induced Pluripotent Stem Cells

Few gene targets of Visual System Homeobox 2 (VSX2) have been identified despite its broad and critical role in the maintenance of neural retina (NR) fate during early retinogenesis. We performed VSX2 ChIP-seq and ChIP-PCR assays on early stage optic vesicle-like structures (OVs) derived from human iPS cells (hiPSCs), which highlighted WNT pathway genes as direct regulatory targets of VSX2. Examination of early NR patterning in hiPSC-OVs from a patient with a functional null mutation in VSX2 revealed mis-expression and upregulation of WNT pathway components and retinal pigmented epithelium (RPE) markers in comparison to control hiPSC-OVs. Furthermore, pharmacological inhibition of WNT signaling rescued the early mutant phenotype, whereas augmentation of WNT signaling in control hiPSC-OVs phenocopied the mutant. These findings reveal an important role for VSX2 as a regulator of WNT signaling and suggest that VSX2 may act to maintain NR identity at the expense of RPE in part by direct repression of WNT pathway constituents. Stem Cells 2016;34:2625-2634.

Chx10 functions as a regulator of molecular pathways controlling the regional identity in the primordial retina

The light-sensitive neural retina (NR) and the retinal pigmented epithelium (RPE) develop from a common primordium, the optic vesicle, raising the question of how they acquire and maintain distinct identities. Here, we demonstrate that sustained misexpression of the Chx10 homeobox gene in the presumptive RPE in chick suppresses accumulation of melanin pigments and promotes ectopic NR-like neural differentiation. This phenotypic change involved ectopic expression of NR transcription factor genes, Sox2, Six3, Rx1 and Optx2, which, when misexpressed, counteracted RPE development without upregulating Chx10. These results suggest that Chx10 can function as a cell autonomous regulator of the regional identity in the primordial retina, presumably through a downstream transcriptional cascade.

Coordinating progenitor cell cycle exit and differentiation in the developing vertebrate retina

The proper development of the vertebrate retina relies heavily on producing the correct number and type of differentiated retinal cell types. To achieve this, proliferating retinal progenitor cells (RPCs) must exit the cell cycle at an appropriate time and correctly express a subset of differentiation markers that help specify retinal cell fate. Homeobox genes, which encode a family of transcription factors, have been accredited to both these processes, implicated in the transcriptional regulation of important cell cycle components, such as cyclins and cyclin-dependent kinases, and proneural genes. This dual regulation of homeobox genes allows these factors to help co-ordinate the transition from the proliferating RPC to postmitotic, differentiated cell. However, understanding the exact molecular targets of these factors remains a challenging task. This commentary highlights the current knowledge we have about how these factors regulate cell cycle progression and differentiation, with particular emphasis on a recent discovery from our lab demonstrating an antagonistic relationship between Vsx2 and Dmbx1 to control RPC proliferation. Future studies should aim to further understand the direct transcriptional targets of these genes, additional co-factors/interacting proteins and the possible recruitment of epigenetic machinery by these homeobox genes.

MicroRNA-20a is essential for normal embryogenesis by targeting vsx1 mRNA in fish

MicroRNAs are major post-transcriptional regulators of gene expression and have essential roles in diverse developmental processes. In vertebrates, some regulatory genes play different roles at different developmental stages. These genes are initially transcribed in a wide embryonic region but restricted within distinct cell types at subsequent stages during development. Therefore, post-transcriptional regulation is required for the transition from one developmental stage to the next and the establishment of different cell identities. However, the regulation of many multiple functional genes at post-transcription level during development remains unknown. Here we show that miR-20a can target the mRNA of vsx1, a multiple functional gene, at the 3'-UTR and inhibit protein expression in both goldfish and zebrafish. The expression of miR-20a is initiated ubiquitously at late gastrula stage and exhibits a tissue-specific pattern in the developing retina. Inhibition of vsx1 3'-UTR mediated protein expression occurs when and where miR-20a is expressed. Decoying miR-20a resulted in severely impaired head, eye and trunk formation in association with excessive generation of vsx1 marked neurons in the spinal cord and defects of somites in the mesoderm region. These results demonstrate that miR-20a is essential for normal embryogenesis by restricting Vsx1 expression in goldfish and zebrafish, and that post-transcriptional regulation is an essential mechanism for Vsx1 playing different roles in diverse developmental processes.

Genetic chimeras reveal the autonomy requirements for Vsx2 in embryonic retinal progenitor cells

BACKGROUND

Vertebrate retinal development is a complex process, requiring the specification and maintenance of retinal identity, proliferative expansion of retinal progenitor cells (RPCs), and their differentiation into retinal neurons and glia. The homeobox gene Vsx2 is expressed in RPCs and required for the proper execution of this retinal program. However, our understanding of the mechanisms by which Vsx2 does this is still rudimentary. To define the autonomy requirements for Vsx2 in the regulation of RPC properties, we generated chimeric mouse embryos comprised of wild-type and Vsx2-deficient cells.

RESULTS

We show that Vsx2 maintains retinal identity in part through the cell-autonomous repression of the retinal pigment epithelium determinant Mitf, and that Lhx2 is required cell autonomously for the ectopic Mitf expression in Vsx2-deficient cells. We also found significant cell-nonautonomous contributions to Vsx2-mediated regulation of RPC proliferation, pointing to an important role for Vsx2 in establishing a growth-promoting extracellular environment. Additionally, we report a cell-autonomous requirement for Vsx2 in controlling when neurogenesis is initiated, indicating that Vsx2 is an important mediator of neurogenic competence. Finally, the distribution of wild-type cells shifted away from RPCs and toward retinal ganglion cell precursors in patches of high Vsx2-deficient cell density to potentially compensate for the lack of fated precursors in these areas.

CONCLUSIONS

Through the generation and analysis of genetic chimeras, we demonstrate that Vsx2 utilizes both cell-autonomous and cell-nonautonomous mechanisms to regulate progenitor properties in the embryonic retina. Importantly, Vsx2's role in regulating Mitf is in part separable from its role in promoting proliferation, and proliferation is excluded as the intrinsic timer that determines when neurogenesis is initiated. These findings highlight the complexity of Vsx2 function during retinal development and provide a framework for identifying the molecular mechanisms mediating these functions.

Mutual antagonism of the paired-type homeobox genes, vsx2 and dmbx1, regulates retinal progenitor cell cycle exit upstream of ccnd1 expression

Understanding the mechanisms that regulate the transition between the proliferative and a post-mitotic state of retinal progenitor cells (RPCs) is key to advancing our knowledge of retinal growth and maturation. In the present study we determined that during zebrafish embryonic retinal neurogenesis, two paired-type homeobox genes - vsx2 and dmbx1 - function in a mutually antagonistic manner. We demonstrate that vsx2 gene expression requires active Fgf signaling and that this in turn suppresses dmbx1 expression and maintains cells in an undifferentiated, proliferative RPC state. This vsx2-dependent RPC state can be prolonged cell-autonomously by knockdown of dmbx1, or it can be suppressed prematurely by the over-expression of dmbx1, which we show can inhibit vsx2 expression and lead to precocious neuronal differentiation. dmbx1 loss of function also results in altered expression of canonical cell cycle genes, and in particular up-regulation of ccnd1, which correlates with our previous finding of a prolonged RPC cell cycle. By knocking down ccnd1 and dmbx1 simultaneously, we show that RPCs can overcome this phenotype to exit the cell cycle on time and differentiate normally into retinal neurons. Collectively, our data provide novel insight into the mechanism that enables RPCs to exit the cell cycle through a previously unrecognized antagonistic interaction of two paired-type homeobox genes that are central regulators of an Fgf-vsx2-dmbx1-ccnd1 signaling axis.

Maternal Vsx1 plays an essential role in regulating prechordal mesendoderm and forebrain formation in zebrafish

Prechordal mesendoderm (PME) is a derivative of gastrula organizer underlying the anterior neural plate of vertebrate embryos. It has been firmly established that PME is critical for head induction and anterior-posterior patterning. Therefore, the establishment of PME in a desired shape and size at a correct position during early embryogenesis is crucial for normal head patterning. However, it remains largely unclear how the desired form and size of PME is generated at a predestined position during early embryogenesis. Here we show that in zebrafish a maternal transcription repressor Vsx1 is essential for this early developmental regulation. Knocking down maternal vsx1 resulted in impaired PME formation and progression associated with a deficient and posteriorized forebrain. Loss- and gain-of-function experiments showed that maternal Vsx1 is essential for repressing ntl ectopic expression in more animal region at early gastrula stages. Chromatin immunoprecipitation assay in combination with core consensus sequence mutation analysis further revealed that maternal Vsx1 can directly repress ntl transcription by binding to the proximal promoter at a specific site. Simultaneous inhibition of ntl function could successfully suppress the defects of both PME and forebrain formation in maternal Vsx1 knockdown embryos. Our results reveal a pivotal role for maternal Vsx1 as a direct transcriptional repressor of ntl expression at the margin of the zebrafish gastrula to ensure directional cell polarization and migration of PME cells.

The role of homeobox genes in retinal development and disease

Homeobox genes are an evolutionarily conserved class of transcription factors that are critical for development of many organ systems, including the brain and eye. During retinogenesis, homeodomain-containing transcription factors, which are encoded by homeobox genes, play essential roles in the regionalization and patterning of the optic neuroepithelium, specification of retinal progenitors and differentiation of all seven of the retinal cell classes that derive from a common progenitor. Homeodomain transcription factors control retinal cell fate by regulating the expression of target genes required for retinal progenitor cell fate decisions and for terminal differentiation of specific retinal cell types. The essential role of homeobox genes during retinal development is demonstrated by the number of human eye diseases, including colobomas and anophthalmia, which are attributed to homeobox gene mutations. In the following review, we highlight the role of homeodomain transcription factors during retinogenesis and regulation of their gene targets. Understanding the complexities of vertebrate retina development will enhance our ability to drive differentiation of specific retinal cell types towards novel cell-based replacement therapies for retinal degenerative diseases.

Loss of MITF expression during human embryonic stem cell differentiation disrupts retinal pigment epithelium development and optic vesicle cell proliferation

Microphthalmia-associated transcription factor (MITF) is a master regulator of pigmented cell survival and differentiation with direct transcriptional links to cell cycle, apoptosis and pigmentation. In mouse, Mitf is expressed early and uniformly in optic vesicle (OV) cells as they evaginate from the developing neural tube, and null Mitf mutations result in microphthalmia and pigmentation defects. However, homozygous mutations in MITF have not been identified in humans; therefore, little is known about its role in human retinogenesis. We used a human embryonic stem cell (hESC) model that recapitulates numerous aspects of retinal development, including OV specification and formation of retinal pigment epithelium (RPE) and neural retina progenitor cells (NRPCs), to investigate the earliest roles of MITF. During hESC differentiation toward a retinal lineage, a subset of MITF isoforms was expressed in a sequence and tissue distribution similar to that observed in mice. In addition, we found that promoters for the MITF-A, -D and -H isoforms were directly targeted by Visual Systems Homeobox 2 (VSX2), a transcription factor involved in patterning the OV toward a NRPC fate. We then manipulated MITF RNA and protein levels at early developmental stages and observed decreased expression of eye field transcription factors, reduced early OV cell proliferation and disrupted RPE maturation. This work provides a foundation for investigating MITF and other highly complex, multi-purposed transcription factors in a dynamic human developmental model system.

Reconciling competence and transcriptional hierarchies with stochasticity in retinal lineages

•

Problems with a strict retinal competence model are explained.

•

The apparent conflict between transcriptional hierarchies and stochasticity is resolved.

•

The underlying nature of retinal progenitor cell stochasticity is discussed.

•

Key issues that can be addressed in the face of stochasticity are enumerated.

Recent advances suggest that there is a stochastic contribution to the proliferation and fate choice of retinal progenitors. How does this stochasticity fit with the progression of temporal competence and the transcriptional hierarchies that also influence cell division and cell fate in the developing retina? Where may stochasticity arise in the system and how do we make progress in this field when we may never fully explain the behavior of individual progenitor cells?

Maternal control of axial-paraxial mesoderm patterning via direct transcriptional repression in zebrafish

Axial-paraxial mesoderm patterning is a special dorsal-ventral patterning event of establishing the vertebrate body plan. Though dorsal-ventral patterning has been extensively studied, the initiation of axial-paraxial mesoderm pattering remains largely unrevealed. In zebrafish, spt cell-autonomously regulates paraxial mesoderm specification and flh represses spt expression to promote axial mesoderm fate, but the expression domains of spt and flh initially overlap in the entire marginal zone of the embryo. Defining spt and flh territories is therefore a premise of axial-paraxial mesoderm patterning. In this study, we investigated why and how the initial expression of flh becomes repressed in the ventrolateral marginal cells during blastula stage. Loss- and gain-of-function experiments showed that a maternal transcription factor Vsx1 is essential for restricting flh expression within the dorsal margin and preserving spt expression and paraxial mesoderm specification in the ventrolateral margin of embryo. Chromatin immunoprecipitation and electrophoretic mobility shift assays in combination with core consensus sequence mutation analysis further revealed that Vsx1 can directly repress flh by binding to the proximal promoter at a specific site. Inhibiting maternal vsx1 translation resulted in confusion of axial and paraxial mesoderm markers expression and axial-paraxial mesoderm patterning. These results demonstrated that direct transcriptional repression of the decisive axial mesoderm gene by maternal ventralizing factor is a crucial regulatory mechanism of initiating axial-paraxial mesoderm patterning in vertebrates.

Blimp1 (Prdm1) prevents re-specification of photoreceptors into retinal bipolar cells by restricting competence

During retinal development, photoreceptors and bipolar cells express the transcription factor Otx2. Blimp1 is transiently expressed in Otx2+ cells. Blimp1 deletion results in excess bipolar cell formation at the expense of photoreceptors. In principle, Blimp1 could be expressed only in Otx2+ cells that are committed to photoreceptor fate. Alternatively, Blimp1 could be expressed broadly in Otx2+ cells and silenced to allow bipolar cell development. To distinguish between these alternatives, we followed the fate of Blimp1 expressing cells using Blimp1-Cre mice and Lox-Stop-Lox reporter strains. We observed that Blimp1+ cells gave rise to all photoreceptors, but also to one third of bipolar cells, consistent with the latter alternative: that Blimp1 inhibits bipolar competence in Otx2+ cells and must be silenced to allow bipolar cell generation. To further test this hypothesis, we looked for transitioning rod photoreceptors in Blimp1 conditional knock-out (CKO) mice carrying the NRL-GFP transgene, which specifically labels rods. Control animals lacked NRL-GFP+ bipolar cells. In contrast, about half of the precociously generated bipolar cells in Blimp1 CKO mice co-expressed GFP, suggesting that rods become re-specified as bipolar cells. Birthdating analyses in control and Blimp1 CKO mice showed that bipolar cells were birthdated as early as E13.5 in Blimp1 CKO mice, five days before this cell type was generated in the wild-type retina. Taken together, our data suggest that early Otx2+ cells upregulate photoreceptor and bipolar genes, existing in a bistable state. Blimp1 likely forms a cross-repressive network with pro-bipolar factors such that the winner of this interaction stabilizes the photoreceptor or bipolar state, respectively.

Lhx2 balances progenitor maintenance with neurogenic output and promotes competence state progression in the developing retina

The LIM-Homeodomain transcription factor Lhx2 is an essential organizer of early eye development and is subsequently expressed in retinal progenitor cells (RPCs). To determine its requirement in RPCs, we performed a temporal series of conditional inactivations in mice with the early RPC driver Pax6 α-Cre and the tamoxifen-inducible Hes1(CreERT2) driver. Deletion of Lhx2 caused a significant reduction of the progenitor population and a corresponding increase in neurogenesis. Precursor fate choice correlated with the time of inactivation; early and late inactivation led to the overproduction of retinal ganglion cells (RGCs) and rod photoreceptors, respectively. In each case, however, the overproduction was selective, occurring at the expense of other cell types and indicating a role for Lhx2 in generating cell type diversity. RPCs that persisted in the absence of Lhx2 continued to generate RGC precursors beyond their normal production window, suggesting that Lhx2 facilitates a transition in competence state. These results identify Lhx2 as a key regulator of RPC properties that contribute to the ordered production of multiple cell types during retinal tissue formation.

Vsx2 controls eye organogenesis and retinal progenitor identity via homeodomain and non-homeodomain residues required for high affinity DNA binding

The homeodomain and adjacent CVC domain in the visual system homeobox (VSX) proteins are conserved from nematodes to humans. Humans with missense mutations in these regions of VSX2 have microphthalmia, suggesting both regions are critical for function. To assess this, we generated the corresponding mutations in mouse Vsx2. The homeodomain mutant protein lacked DNA binding activity and the knock-in mutant phenocopied the null mutant, ocular retardation J. The CVC mutant protein exhibited weakened DNA binding; and, although the corresponding knock-in allele was recessive, it unexpectedly caused the strongest phenotype, as indicated by severe microphthalmia and hyperpigmentation of the neural retina. This occurred through a cryptic transcriptional feedback loop involving the transcription factors Mitf and Otx1 and the Cdk inhibitor p27^Kip1. Our data suggest that the phenotypic severity of the CVC mutant depends on the weakened DNA binding activity elicited by the CVC mutation and a previously unknown protein interaction between Vsx2 and its regulatory target Mitf. Our data also suggest that an essential function of the CVC domain is to assist the homeodomain in high-affinity DNA binding, which is required for eye organogenesis and unhindered execution of the retinal progenitor program in mammals. Finally, the genetic and phenotypic behaviors of the CVC mutation suggest it has the characteristics of a recessive neomorph, a rare type of genetic allele.

Problems with the early development of the mammalian retina can cause congenital eye defects such as microphthalmia, in which the eye is dramatically smaller and functionally compromised. Severe microphthalmia is associated with mutations in the retinal-expressed visual system homeobox 2 (Vsx2) gene, but how Vsx2 controls retinal development, and ultimately eye formation, has remained unclear. We assessed the impact of two missense mutations, discovered in humans, on Vsx2 function and eye development in mice. One mutation altered a highly conserved residue of the homeodomain, and the other altered a highly conserved residue in the CVC domain, a region of unresolved function. Both mutations impacted the DNA binding properties of the protein, although to differing extents. Likewise, both mutations caused microphthalmia and disruptions in retinal development, also to differing extents and by distinct mechanisms. Our data suggest that Vsx2 acts as a gatekeeper of the retinal gene expression program by preventing the activation of interfering or competing gene expression programs. We propose that the evolutionary stable association between the VSX-class homeodomain and CVC domain set the stage for Vsx2 or its archetype to assume a gatekeeper function for retinal development and ultimately eye organogenesis.

Cyclin D1 inactivation extends proliferation and alters histogenesis in the postnatal mouse retina

BACKGROUND

The cell-cycle regulator Cyclin D1 is expressed in embryonic retinal progenitor cells (RPCs) and regulates their cell-cycle rate and neurogenic output. We report here that Cyclin D1 also has important functions in postnatal retinal histogenesis.

RESULTS

The initial production of Müller glia and bipolar cells was enhanced in Cyclin D1 knockout (Ccnd1(-/-) ) retinas. Despite a steeper than normal rate of depletion of the RPC population at embryonic ages, postnatal Ccnd1(-/-) retinas exhibited an extended window of proliferation, neurogenesis, and gliogenesis. Cyclin D3, normally confined to Müller glia, was prematurely expressed in Ccnd1(-/-) RPCs. However, Cyclin D3 did not compensate for Cyclin D1 in regulating cell-cycle kinetics or neurogenic output.

CONCLUSIONS

The data presented in this study along with our previous finding that Cyclin D2 was unable to completely compensate for the absence of Cyclin D1 indicate that Cyclin D1 regulates retinal histogenesis in ways not shared by the other D-cyclins.

Cell fate determination in the vertebrate retina

The vertebrate retina is a well-characterized and tractable model for studying neurogenesis. Retinal neurons and glia are generated in a conserved sequence from a pool of multipotent progenitor cells, and numerous cell fate determinants for the different classes of retinal cell types have been identified. Here, we summarize several recent developments in the field that have advanced understanding of the regulation of multipotentiality and temporal competence of progenitors. We also discuss recent insights into the relative influence of lineage-based versus stochastic modes of cell fate determination. Enhancing and integrating knowledge of the molecular and genetic machinery underlying retinal development is critically important for understanding not only normal developmental mechanisms, but also therapeutic interventions aimed at restoring vision loss.

Sequence divergence in the 3'-untranslated region has an effect on the subfunctionalization of duplicate genes

Recent studies demonstrated that sequence divergence in both transcriptional regulatory region and coding region contributes to the subfunctionalization of duplicate gene. However, whether sequence divergence in the 3'-untranslated region (3'-UTR) has an impact on the subfunctionalization of duplicate genes remains unclear. Here, we identified two diverging duplicate vsx1 (visual system homeobox-1) loci in goldfish, named vsx1A1 and vsx1A2. Phylogenetic analysis suggests that vsx1A1 and vsx1A2 may arise from a duplication of vsx1 after the separation of goldfish and zebrafish. Sequence comparison revealed that divergence in both transcriptional and translational regulatory regions is higher than divergence in the introns. vsx1A2 expresses during blastula and gastrula stages and in adult retina but silences from segmentation stage to hatching stage, vsx1A1 starts expression from segmentation onward. Comparing to that zebrafish vsx1 expresses in all the developmental stages and in the adult retina, it appears that goldfish vsx1A1 and vsx1A2 are under going to share the functions of ancestral vsx1. The different but overlapping temporal expression patterns of vsx1A1 and vsx1A2 suggest that sequence divergence in the promoter region of duplicate vsx1 is not sufficient for partitioning the functions of ancestral vsx1. By comparing vsx1A1 and vsx1A2 3'-UTR-linked green fluorescent protein gene expression patterns, we demonstrated that the 3'-UTR of vsx1A1 remains but the 3'-UTR of vsx1A2 has lost the capability of mediating bipolar cell specific expression during retina development. These results indicate that sequence divergence in the 3'-UTRs has a clear effect on subfunctionalization of the duplicate genes.

Enhanced generation of retinal progenitor cells from human retinal pigment epithelial cells induced by amniotic fluid

BACKGROUND

Retinal progenitor cells are a convenient source of cell replacement therapy in retinal degenerative disorders. The purpose of this study was to evaluate the expression patterns of the homeobox genes PAX6 and CHX10 (retinal progenitor markers) during treatment of human retinal pigment epithelium (RPE) cells with amniotic fluid (AF), RPE cells harvested from neonatal cadaver globes were cultured in a mixture of DMEM and Ham's F12 supplemented with 10% FBS. At different passages, cells were trypsinized and co-cultured with 30% AF obtained from normal fetuses of 1416 weeks gestational age.

RESULTS

Compared to FBS-treated controls, AF-treated cultures exhibited special morphological changes in culture, including appearance of spheroid colonies, improved initial cell adhesion and ordered cell alignment. Cell proliferation assays indicated a remarkable increase in the proliferation rate of RPE cells cultivated in 30% AF-supplemented medium, compared with those grown in the absence of AF. Immunocytochemical analyses exhibited nuclear localization of retinal progenitor markers at a ratio of 33% and 27% for CHX10 and PAX6, respectively. This indicated a 3-fold increase in retinal progenitor markers in AF-treated cultures compared to FBS-treated controls. Real-time PCR data of retinal progenitor genes (PAX6, CHX10 and VSX-1) confirmed these results and demonstrated AF's capacity for promoting retinal progenitor cell generation.

CONCLUSION

Taken together, the results suggest that AF significantly promotes the rate of retinal progenitor cell generation, indicating that AF can be used as an enriched supplement for serum-free media used for the in vitro propagation of human progenitor cells.

Vsx1 regulates terminal differentiation of type 7 ON bipolar cells

Although retinal bipolar cells represent a morphologically well defined population of retinal interneurons, very little is known about the developmental mechanisms that regulate their processing. Furthermore, the identity of specific bipolar cell types that function in distinct visual circuits remains poorly understood. Here, we show that the homeobox gene Vsx1 is expressed in Type 7 ON bipolar cells. In the absence of Vsx1, Type 7 bipolar cells exhibit proper morphological specification but show defects in terminal gene expression. Vsx1 is required for the repression of bipolar cell-specific markers, including Calcium-binding protein 5 and Chx10. This contrasts its genetic requirement as an activator of gene expression in OFF bipolar cells. To assess possible ON signaling defects in Vsx1-null mice, we recorded specifically from ON-OFF directionally selective ganglion cells (DSGCs), which cofasciculate with Type 7 bipolar cell terminals. Vsx1-null ON-OFF DSGCs received more sustained excitatory synaptic input, possibly due to Type 7 bipolar cell defects. Interestingly, in Vsx1-null mice, the directionally selective circuit is functional but compromised. Together, these findings indicate that Vsx1 regulates terminal gene expression in Type 7 bipolar cells and is necessary for proper ON visual signaling within a directionally selective circuit.

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.

Bipolar cells in the zebrafish retina

Zebrafish are an existing model for genetic and developmental studies due to their rapid external development and transparent embryos, which allow easy manipulation and observation of early developmental stages. The application of the zebrafish model to vision research has allowed for examination of retinal development and the characteristics of different retinal cell types, including bipolar cells. In particular, bipolar cell development, including differentiation, maturation, and gene expression, has been documented, as has physiological properties, such as voltage- and ligand-gated currents, and neurotransmitter receptor and ion channel expression. Mutant strains and transgenic lines have been used to document how bipolar cell connections and/or development may be altered, and toxicological studies examining how environmental factors may impact bipolar cell activity have been performed. The purpose of this paper was to review the existing literature on zebrafish bipolar cells, to provide a comprehensive overview of current information pertaining to this retinal cell type.

Blimp1 controls photoreceptor versus bipolar cell fate choice during retinal development

Photoreceptors, rods and cones are the most abundant cell type in the mammalian retina. However, the molecules that control their development are not fully understood. In studies of photoreceptor fate determination, we found that Blimp1 (Prdm1) is expressed transiently in developing photoreceptors. We analyzed the function of Blimp1 in the mouse retina using a conditional deletion approach. Developmental analysis of mutants showed that Otx2(+) photoreceptor precursors ectopically express the bipolar cell markers Chx10 (Vsx2) and Vsx1, adopting bipolar instead of photoreceptor fate. However, this fate shift did not occur until the time when bipolar cells are normally specified during development. Most of the excess bipolar cells died around the time of bipolar cell maturation. Our results suggest that Blimp1 expression stabilizes immature photoreceptors by preventing bipolar cell induction. We conclude that Blimp1 regulates the decision between photoreceptor and bipolar cell fates in the Otx2(+) cell population during retinal development.

Lhx2 links the intrinsic and extrinsic factors that control optic cup formation

A crucial step in eye organogenesis is the transition of the optic vesicle into the optic cup. Several transcription factors and extracellular signals mediate this transition, but whether a single factor links them into a common genetic network is unclear. Here, we provide evidence that the LIM homeobox gene Lhx2, which is expressed in the optic neuroepithelium, fulfils such a role. In Lhx2(-/-) mouse embryos, eye field specification and optic vesicle morphogenesis occur, but development arrests prior to optic cup formation in both the optic neuroepithelium and lens ectoderm. This is accompanied by failure to maintain or initiate the expression patterns of optic-vesicle-patterning and lens-inducing determinants. Of the signaling pathways examined, only BMP signaling is noticeably altered and Bmp4 and Bmp7 mRNAs are undetectable. Lhx2(-/-) optic vesicles and lens ectoderm upregulate Pax2, Fgf15 and Sox2 in response to BMP treatments, and Lhx2 genetic mosaics reveal that transcription factors, including Vsx2 and Mitf, require Lhx2 cell-autonomously for their expression. Our data indicate that Lhx2 is required for optic vesicle patterning and lens formation in part by regulating BMP signaling in an autocrine manner in the optic neuroepithelium and in a paracrine manner in the lens ectoderm. We propose a model in which Lhx2 is a central link in a genetic network that coordinates the multiple pathways leading to optic cup formation.

The homeobox gene CHX10/VSX2 regulates RdCVF promoter activity in the inner retina

Rod-derived Cone Viability Factor (RdCVF) is a trophic factor with therapeutic potential for the treatment of retinitis pigmentosa, a retinal disease that commonly results in blindness. RdCVF is encoded by Nucleoredoxin-like 1 (Nxnl1), a gene homologous with the family of thioredoxins that participate in the defense against oxidative stress. RdCVF expression is lost after rod degeneration in the first phase of retinitis pigmentosa, and this loss has been implicated in the more clinically significant secondary cone degeneration that often occurs. Here, we describe a study of the Nxnl1 promoter using an approach that combines promoter and transcriptomic analysis. By transfection of selected candidate transcription factors, chosen based upon their expression pattern, we identified the homeodomain proteins CHX10/VSX2, VSX1 and PAX4, as well as the zinc finger protein SP3, as factors that can stimulate both the mouse and human Nxnl1 promoter. In addition, CHX10/VSX2 binds to the Nxnl1 promoter in vivo. Since CHX10/VSX2 is expressed predominantly in the inner retina, this finding motivated us to demonstrate that RdCVF is expressed in the inner as well as the outer retina. Interestingly, the loss of rods in the rd1 mouse, a model of retinitis pigmentosa, is associated with decreased expression of RdCVF by inner retinal cells as well as by rods. Based upon these results, we propose an alternative therapeutic strategy aimed at recapitulating RdCVF expression in the inner retina, where cell loss is not significant, to prevent secondary cone death and central vision loss in patients suffering from retinitis pigmentosa.

Beta-catenin controls differentiation of the retinal pigment epithelium in the mouse optic cup by regulating Mitf and Otx2 expression

The retinal pigment epithelium (RPE) consists of a monolayer of cuboidal, pigmented cells that is located between the retina and the choroid. The RPE is vital for growth and function of the vertebrate eye and improper development results in congenital defects, such as microphthalmia or anophthalmia, or a change of cell fate into neural retina called transdifferentiation. The transcription factors microphthalmia-associated transcription factor (Mitf) and orthodenticle homolog 2 (Otx2) are crucial for RPE development and function; however, very little is known about their regulation. Here, by using a Wnt-responsive reporter, we show that the Wnt/beta-catenin pathway is activated in the differentiating mouse RPE. Cre-mediated, RPE-specific disruption of beta-catenin after the onset of RPE specification causes severe defects, resulting in microphthalmia with coloboma, disturbed lamination, and mislocalization of adherens junction proteins. Upon beta-catenin deletion, the RPE transforms into a multilayered tissue in which the expression of Mitf and Otx2 is downregulated, while retina-specific gene expression is induced, which results in the transdifferentiation of RPE into retina. Chromatin immunoprecipitation (ChIP) and luciferase assays indicate that beta-catenin binds near to and activates potential TCF/LEF sites in the Mitf and Otx2 enhancers. We conclude that Wnt/beta-catenin signaling is required for differentiation of the RPE by directly regulating the expression of Mitf and Otx2. Our study is the first to show that an extracellular signaling pathway directly regulates the expression of RPE-specific genes such as Mitf and Otx2, and elucidates a new role for the Wnt/beta-catenin pathway in organ formation and development.

Cyclin D1 fine-tunes the neurogenic output of embryonic retinal progenitor cells

BACKGROUND

Maintaining the correct balance of proliferation versus differentiation in retinal progenitor cells (RPCs) is essential for proper development of the retina. The cell cycle regulator cyclin D1 is expressed in RPCs, and mice with a targeted null allele at the cyclin D1 locus (Ccnd1-/-) have microphthalmia and hypocellular retinas, the latter phenotype attributed to reduced RPC proliferation and increased photoreceptor cell death during the postnatal period. How cyclin D1 influences RPC behavior, especially during the embryonic period, is unclear.

RESULTS

In this study, we show that embryonic RPCs lacking cyclin D1 progress through the cell cycle at a slower rate and exit the cell cycle at a faster rate. Consistent with enhanced cell cycle exit, the relative proportions of cell types born in the embryonic period, such as retinal ganglion cells and photoreceptor cells, are increased. Unexpectedly, cyclin D1 deficiency decreases the proportions of other early born retinal neurons, namely horizontal cells and specific amacrine cell types. We also found that the laminar positioning of horizontal cells and other cell types is altered in the absence of cyclin D1. Genetically replacing cyclin D1 with cyclin D2 is not efficient at correcting the phenotypes due to the cyclin D1 deficiency, which suggests the D-cyclins are not fully redundant. Replacement with cyclin E or inactivation of cyclin-dependent kinase inhibitor p27Kip1 restores the balance of RPCs and retinal cell types to more normal distributions, which suggests that regulation of the retinoblastoma pathway is an important function for cyclin D1 during embryonic retinal development.

CONCLUSION

Our findings show that cyclin D1 has important roles in RPC cell cycle regulation and retinal histogenesis. The reduction in the RPC population due to a longer cell cycle time and to an enhanced rate of cell cycle exit are likely to be the primary factors driving retinal hypocellularity and altered output of precursor populations in the embryonic Ccnd1-/- retina.

Vsx2 in the zebrafish retina: restricted lineages through derepression

BACKGROUND

The neurons in the vertebrate retina arise from multipotent retinal progenitor cells (RPCs). It is not clear, however, which progenitors are multipotent or why they are multipotent.

RESULTS

In this study we show that the homeodomain transcription factor Vsx2 is initially expressed throughout the retinal epithelium, but later it is downregulated in all but a minor population of bipolar cells and all Müller glia. The Vsx2-negative daughters of Vsx2-positive RPCs divide and give rise to all other cell types in the retina. Vsx2 is a repressor whose targets include transcription factors such as Vsx1, which is expressed in the progenitors of distinct non-Vsx2 bipolars, and the basic helix-loop-helix transcription factor Ath5, which restricts the fate of progenitors to retinal ganglion cells, horizontal cells, amacrine cells and photoreceptors fates. Foxn4, expressed in the progenitors of amacrine and horizontal cells, is also negatively regulated by Vsx2.

CONCLUSION

Our data thus suggest Vsx2-positive RPCs are fully multipotent retinal progenitors and that when Vsx2 is downregulated, Vsx2-negative progenitors escape Vsx2 repression and so are able to express factors that restrict lineage potential.