Forkhead Foxe3 maps to the dysgenetic lens locus and is critical in lens development and differentiation

Through the Cat-Map Gateway: A Brief History of Cataract Genetics

Clouding of the transparent eye lens, or cataract(s), is a leading cause of visual impairment that requires surgical replacement with a synthetic intraocular lens to effectively restore clear vision. Most frequently, cataract is acquired with aging as a multifactorial or complex trait. Cataract may also be inherited as a classic Mendelian trait-often with an early or pediatric onset-with or without other ocular and/or systemic features. Since the early 1990s, over 85 genes and loci have been genetically associated with inherited and/or age-related forms of cataract. While many of these underlying genes-including those for lens crystallins, connexins, and transcription factors-recapitulate signature features of lens development and differentiation, an increasing cohort of unpredicted genes, including those involved in cell-signaling, membrane remodeling, and autophagy, has emerged-providing new insights regarding lens homeostasis and aging. This review provides a brief history of gene discovery for inherited and age-related forms of cataract compiled in the Cat-Map database and highlights potential gene-based therapeutic approaches to delay, reverse, or even prevent cataract formation that may help to reduce the increasing demand for cataract surgery.

ATF4 May Be Essential for Adaption of the Ocular Lens to Its Avascular Environment

The late embryonic mouse lens requires the transcription factor ATF4 for its survival although the underlying mechanisms were unknown. Here, RNAseq analysis revealed that E16.5 Atf4 null mouse lenses downregulate the mRNA levels of lens epithelial markers as well as known markers of late lens fiber cell differentiation. However, a comparison of this list of differentially expressed genes (DEGs) with other known transcriptional regulators of lens development indicated that ATF4 expression is not directly controlled by the previously described lens gene regulatory network. Pathway analysis revealed that the Atf4 DEG list was enriched in numerous genes involved in nutrient transport, amino acid biosynthesis, and tRNA charging. These changes in gene expression likely result in the observed reductions in lens free amino acid and glutathione levels, which would result in the observed low levels of extractable lens protein, finally leading to perinatal lens disintegration. These data demonstrate that ATF4, via its function in the integrated stress response, is likely to play a crucial role in mediating the adaption of the lens to the avascularity needed to maintain lens transparency.

Integrated single-cell multiomics uncovers foundational regulatory mechanisms of lens development and pathology

Ocular lens development entails epithelial to fiber cell differentiation, defects in which cause congenital cataract. We report the first single-cell multiomic atlas of lens development, leveraging snRNA-seq, snATAC-seq, and CUT&RUN-seq to discover novel mechanisms of cell fate determination and cataract-linked regulatory networks. A comprehensive profile of cis- and trans-regulatory interactions, including for the cataract-linked transcription factor MAF, is established across a temporal trajectory of fiber cell differentiation. Further, we divulge a conserved epigenetic paradigm of cellular differentiation, defined by progressive loss of H3K27 methylation writer Polycomb repressive complex 2 (PRC2). PRC2 localizes to heterochromatin domains across master-regulator transcription factor gene bodies, suggesting it safeguards epithelial cell fate. Moreover, we demonstrate that FGF hyper-stimulation in vivo leads to MAF network activation and the emergence of novel lens cell states. Collectively, these data depict a comprehensive portrait of lens fiber cell differentiation, while defining regulatory effectors of cell identity and cataract formation.

Comprehensive phenotypic and functional analysis of dominant and recessive FOXE3 alleles in ocular developmental disorders

The forkhead transcription factor FOXE3 is critical for vertebrate eye development. Recessive and dominant variants cause human ocular disease but the full range of phenotypes and mechanisms of action for the two classes of variants are unknown. We identified FOXE3 variants in individuals with congenital eye malformations and carried out in vitro functional analysis on selected alleles. Sixteen new recessive and dominant families, including six novel variants, were identified. Analysis of new and previously reported genetic and clinical data demonstrated a broad phenotypic range with an overlap between recessive and dominant disease. Most families with recessive alleles, composed of truncating and forkhead-domain missense variants, had severe corneal opacity (90%; sclerocornea in 47%), aphakia (83%) and microphthalmia (80%), but some had milder features including isolated cataract. The phenotype was most variable for recessive missense variants, suggesting that the functional consequences may be highly dependent on the type of amino acid substitution and its position. When assessed, aniridia or iris hypoplasia were noted in 89% and optic nerve anomalies in 60% of recessive cases, indicating that these defects are also common and may be underrecognized. In dominant pedigrees, caused by extension variants, normal eye size (96%), cataracts (99%) and variable anterior segment anomalies were seen in most, but some individuals had microphthalmia, aphakia or sclerocornea, more typical of recessive disease. Functional studies identified variable effects on the protein stability, DNA binding, nuclear localization and transcriptional activity for recessive FOXE3 variants, whereas dominant alleles showed severe impairment in all areas and dominant-negative characteristics.

Insights into the regulatory molecules involved in glaucoma pathogenesis

Glaucoma is an important cause of irreversible blindness, characterized by optic nerve anomalies. Increased intraocular pressure (IOP) and aging are major risk factors. Retinal ganglion cells and trabecular meshwork cells are certainly involved in the etiology of glaucoma. Glaucoma is usually a complex disease, and various genes and functions may contribute to its etiology. Among these may be genes that encode regulatory molecules. In this review, regulatory molecules including 18 transcription factors (TFs), 195 microRNAs (miRNAs), 106 long noncoding RNAs (lncRNAs), and two circular RNAs (circRNAs) that are reasonable candidates for having roles in glaucoma pathogenesis are described. The targets of the regulators are reported. Glaucoma-related features including apoptosis, stress responses, immune functions, ECM properties, IOP, and eye development are affected by the targeted genes. The targeted genes that are frequently targeted by multiple regulators most often affect apoptosis and the related features of cell death and cell survival. BCL2, CDKN1A, and TP53 are among the frequent targets of three types of glaucoma-relevant regulators, TFs, miRNAs, and lncRNAs. TP53 was itself identified as a glaucoma-relevant TF. Several of the glaucoma-relevant TFs are themselves among frequent targets of regulatory molecules, which is consistent with existence of a complex network involved in glaucoma pathogenesis.

Identification and expression profiles of Fox transcription factors in the Yesso scallop (Patinopecten yessoensis)

The forkhead box (Fox) gene family is a family of transcription factors that play important roles in a variety of biological processes in vertebrates, including early development and cell proliferation and differentiation. However, at present, studies on the mollusk Fox family are relatively lacking. In the present study, the Fox gene family of the Yesso scallop (Patinopecten yessoensis) was systematically identified. In addition, the expression profiles of the Fox gene family in early development and adult tissues were analyzed. The results showed that there were 26 Fox genes in P. yessoensis. Of the 26 genes, 24 belonged to 20 subfamilies. The Fox genes belonging to the I, Q1, R and S subfamilies were absent in P. yessoensis. The other 2 genes formed 2 independent clades with the Fox genes of other mollusks and protostomes. They might be new members of the Fox family and were named FoxY and FoxZ. P. yessoensis contained a FoxC-FoxL1 gene cluster similar in structure to that of Branchiostoma floridae, suggesting that the cluster might already exist in the ancestors of bilaterally symmetrical animals. The gene expression analysis of Fox showed that most of the genes were continuously expressed in multiple stages of early development, suggesting that Fox genes might be widely involved in the regulation of embryo and larval development of P. yessoensis. Nine Fox genes were specifically expressed in certain tissues, such as the nerve ganglia, foot, ovary, testis, and gills. For the 9 genes that were differentially expressed between the testis and ovary, their expression levels were analyzed during the 4 developmental stages of gonads. The results showed that FoxL2, FoxE and FoxY were highly expressed in the ovary during all developmental stages, while FoxZ was highly expressed in the testis during all developmental stages. The results suggested that these genes might play an important role in sex maintenance or gametogenesis. The present study could provide a reference for evolutionary and functional studies of the Fox family in metazoans.

GLI3 repressor but not GLI3 activator is essential for mouse eye patterning and morphogenesis

Since 1967, it is known that the loss of GLI3 causes very severe defects in murine eye development. GLI3 is able to act as a transcriptional activator (GLI3-A) or as a transcriptional repressor (GLI3-R). Soon after the discovery of these GLI3 isoforms, the question arose which of the different isoforms is involved in eye formation - GLI3-A, GLI3-R or even both. For several years, this question remained elusive. By analysing the eye morphogenesis of Gli3^XtJ/XtJ mouse embryos that lack GLI3-A and GLI3-R and of Gli3^Δ699/Δ699 mouse embryos in which only GLI3-A is missing, we revealed that GLI3-A is dispensable in vertebrate eye formation. Remarkably, our study shows that GLI3-R is sufficient for the creation of morphologically normal eyes although the molecular setup deviates substantially from normality. In depth-investigations elucidated that GLI3-R controls numerous key players in eye development and governs lens and retina development at least partially via regulating WNT/β-CATENIN signalling.

Presenilin gene function and Notch signaling feedback regulation in the developing mouse lens

Presenilins (Psen1 and Psen2 in mice) are polytopic transmembrane proteins that act in the γ-secretase complex to make intra-membrane cleavages of their substrates, including the well-studied Notch receptors. Such processing releases the Notch intracellular domain, allowing it to physically relocate from the cell membrane to the nucleus where it acts in a transcriptional activating complex to regulate downstream genes in the signal-receiving cell. Previous studies of Notch pathway mutants for Jagged1, Notch2, and Rbpj demonstrated that canonical signaling is a necessary component of normal mouse lens development. However, the central role of Psens within the γ-secretase complex has never been explored in any developing eye tissue or cell type. By directly comparing Psen single and double mutant phenotypes during mouse lens development, we found a stronger requirement for Psen1, although both genes are needed for progenitor cell growth and to prevent apoptosis. We also uncovered a novel genetic interaction between Psen1 and Jagged1. By quantifying protein and mRNA levels of key Notch pathway genes in Psen1/2 or Jagged1 mutant lenses, we identified multiple points in the overall signaling cascade where feedback regulation can occur. Our data are consistent with the loss of particular genes indirectly influencing the transcription level of another. However, we conclude that regulating Notch2 protein levels is particularly important during normal signaling, supporting the importance of post-translational regulatory mechanisms in this tissue.

A zebrafish model of foxe3 deficiency demonstrates lens and eye defects with dysregulation of key genes involved in cataract formation in humans

The Forkhead box E3 (FOXE3) gene encodes a transcription factor with a forkhead/winged helix domain that is critical for development of the lens and anterior segment of the eye. Monoallelic and biallelic deleterious sequence variants in FOXE3 cause aphakia, cataracts, sclerocornea and microphthalmia in humans. We used clustered regularly interspaced short palindromic repeats/Cas9 injections to target the foxe3 transcript in zebrafish in order to create an experimental model of loss of function for this gene. Larvae that were homozygous for an indel variant, c.296_300delTGCAG, predicting p.(Val99Alafs*2), demonstrated severe eye defects, including small or absent lenses and microphthalmia. The lenses of the homozygous foxe3 indel mutants showed more intense staining with zl-1 antibody compared to control lenses, consistent with increased lens fiber cell differentiation. Whole genome transcriptome analysis (RNA-Seq) on RNA isolated from wildtype larvae and larvae with eye defects that were putative homozygotes for the foxe3 indel variant found significant dysregulation of genes expressed in the lens and eye whose orthologues are associated with cataracts in human patients, including cryba2a, cryba1l1, mipa and hsf4. Comparative analysis of this RNA-seq data with iSyTE data identified several lens-enriched genes to be down-regulated in foxe3 indel mutants. We also noted upregulation of lgsn and crygmxl2 and downregulation of fmodb and cx43.4, genes that are expressed in the zebrafish lens, but that are not yet associated with an eye phenotype in humans. These findings demonstrate that this new zebrafish foxe3 mutant model is highly relevant to the study of the gene regulatory networks conserved in vertebrate lens and eye development.

Mutation update of transcription factor genes FOXE3, HSF4, MAF, and PITX3 causing cataracts and other developmental ocular defects

Mutations in the transcription factor genes FOXE3, HSF4, MAF, and PITX3 cause congenital lens defects including cataracts that may be accompanied by defects in other components of the eye or in nonocular tissues. We comprehensively describe here all the variants in FOXE3, HSF4, MAF, and PITX3 genes linked to human developmental defects. A total of 52 variants for FOXE3, 18 variants for HSF4, 20 variants for MAF, and 19 variants for PITX3 identified so far in isolated cases or within families are documented. This effort reveals FOXE3, HSF4, MAF, and PITX3 to have 33, 16, 18, and 7 unique causal mutations, respectively. Loss-of-function mutant animals for these genes have served to model the pathobiology of the associated human defects, and we discuss the currently known molecular function of these genes, particularly with emphasis on their role in ocular development. Finally, we make the detailed FOXE3, HSF4, MAF, and PITX3 variant information available in the Leiden Online Variation Database (LOVD) platform at https://www.LOVD.nl/FOXE3, https://www.LOVD.nl/HSF4, https://www.LOVD.nl/MAF, and https://www.LOVD.nl/PITX3. Thus, this article informs on key variants in transcription factor genes linked to cataract, aphakia, corneal opacity, glaucoma, microcornea, microphthalmia, anterior segment mesenchymal dysgenesis, and Ayme-Gripp syndrome, and facilitates their access through Web-based databases.

Chromatin remodeling enzyme Snf2h regulates embryonic lens differentiation and denucleation

Ocular lens morphogenesis is a model for investigating mechanisms of cellular differentiation, spatial and temporal gene expression control, and chromatin regulation. Brg1 (Smarca4) and Snf2h (Smarca5) are catalytic subunits of distinct ATP-dependent chromatin remodeling complexes implicated in transcriptional regulation. Previous studies have shown that Brg1 regulates both lens fiber cell differentiation and organized degradation of their nuclei (denucleation). Here, we employed a conditional Snf2h(flox) mouse model to probe the cellular and molecular mechanisms of lens formation. Depletion of Snf2h induces premature and expanded differentiation of lens precursor cells forming the lens vesicle, implicating Snf2h as a key regulator of lens vesicle polarity through spatial control of Prox1, Jag1, p27(Kip1) (Cdkn1b) and p57(Kip2) (Cdkn1c) gene expression. The abnormal Snf2h(-/-) fiber cells also retain their nuclei. RNA profiling of Snf2h(-/) (-) and Brg1(-/-) eyes revealed differences in multiple transcripts, including prominent downregulation of those encoding Hsf4 and DNase IIβ, which are implicated in the denucleation process. In summary, our data suggest that Snf2h is essential for the establishment of lens vesicle polarity, partitioning of prospective lens epithelial and fiber cell compartments, lens fiber cell differentiation, and lens fiber cell nuclear degradation.

Systems biology of lens development: A paradigm for disease gene discovery in the eye

Over the past several decades, the biology of the developing lens has been investigated using molecular genetics-based approaches in various vertebrate model systems. These efforts, involving target gene knockouts or knockdowns, have led to major advances in our understanding of lens morphogenesis and the pathological basis of cataracts, as well as of other lens related eye defects. In particular, we now have a functional understanding of regulators such as Pax6, Six3, Sox2, Oct1 (Pou2f1), Meis1, Pnox1, Zeb2 (Sip1), Mab21l1, Foxe3, Tfap2a (Ap2-alpha), Pitx3, Sox11, Prox1, Sox1, c-Maf, Mafg, Mafk, Hsf4, Fgfrs, Bmp7, and Tdrd7 in this tissue. However, whether these individual regulators interact or their targets overlap, and the significance of such interactions during lens morphogenesis, is not well defined. The arrival of high-throughput approaches for gene expression profiling (microarrays, RNA-sequencing (RNA-seq), etc.), which can be coupled with chromatin immunoprecipitation (ChIP) or RNA immunoprecipitation (RIP) assays, along with improved computational resources and publically available datasets (e.g. those containing comprehensive protein-protein, protein-DNA information), presents new opportunities to advance our understanding of the lens tissue on a global systems level. Such systems-level knowledge will lead to the derivation of the underlying lens gene regulatory network (GRN), defined as a circuit map of the regulator-target interactions functional in lens development, which can be applied to expedite cataract gene discovery. In this review, we cover the various systems-level approaches such as microarrays, RNA-seq, and ChIP that are already being applied to lens studies and discuss strategies for assembling and interpreting these vast amounts of high-throughput information for effective dispersion to the scientific community. In particular, we discuss strategies for effective interpretation of this new information in the context of the rich knowledge obtained through the application of traditional single-gene focused experiments on the lens. Finally, we discuss our vision for integrating these diverse high-throughput datasets in a single web-based user-friendly tool iSyTE (integrated Systems Tool for Eye gene discovery) - a resource that is already proving effective in the identification and characterization of genes linked to lens development and cataract. We anticipate that application of a similar approach to other ocular tissues such as the retina and the cornea, and even other organ systems, will significantly impact disease gene discovery.

FOXE3 mutations predispose to thoracic aortic aneurysms and dissections

The ascending thoracic aorta is designed to withstand biomechanical forces from pulsatile blood. Thoracic aortic aneurysms and acute aortic dissections (TAADs) occur as a result of genetically triggered defects in aortic structure and a dysfunctional response to these forces. Here, we describe mutations in the forkhead transcription factor FOXE3 that predispose mutation-bearing individuals to TAAD. We performed exome sequencing of a large family with multiple members with TAADs and identified a rare variant in FOXE3 with an altered amino acid in the DNA-binding domain (p.Asp153His) that segregated with disease in this family. Additional pathogenic FOXE3 variants were identified in unrelated TAAD families. In mice, Foxe3 deficiency reduced smooth muscle cell (SMC) density and impaired SMC differentiation in the ascending aorta. Foxe3 expression was induced in aortic SMCs after transverse aortic constriction, and Foxe3 deficiency increased SMC apoptosis and ascending aortic rupture with increased aortic pressure. These phenotypes were rescued by inhibiting p53 activity, either by administration of a p53 inhibitor (pifithrin-α), or by crossing Foxe3-/- mice with p53-/- mice. Our data demonstrate that FOXE3 mutations lead to a reduced number of aortic SMCs during development and increased SMC apoptosis in the ascending aorta in response to increased biomechanical forces, thus defining an additional molecular pathway that leads to familial thoracic aortic disease.

Conserved genetic pathways associated with microphthalmia, anophthalmia, and coloboma

The human eye is a complex organ whose development requires extraordinary coordination of developmental processes. The conservation of ocular developmental steps in vertebrates suggests possible common genetic mechanisms. Genetic diseases involving the eye represent a leading cause of blindness in children and adults. During the last decades, there has been an exponential increase in genetic studies of ocular disorders. In this review, we summarize current success in identification of genes responsible for microphthalmia, anophthalmia, and coloboma (MAC) phenotypes, which are associated with early defects in embryonic eye development. Studies in animal models for the orthologous genes identified overlapping phenotypes for most factors, confirming the conservation of their function in vertebrate development. These animal models allow for further investigation of the mechanisms of MAC, integration of various identified genes into common developmental pathways and finally, provide an avenue for the development and testing of therapeutic interventions.

Transcriptional regulation of cranial sensory placode development

Cranial sensory placodes derive from discrete patches of the head ectoderm and give rise to numerous sensory structures. During gastrulation, a specialized "neural border zone" forms around the neural plate in response to interactions between the neural and nonneural ectoderm and signals from adjacent mesodermal and/or endodermal tissues. This zone subsequently gives rise to two distinct precursor populations of the peripheral nervous system: the neural crest and the preplacodal ectoderm (PPE). The PPE is a common field from which all cranial sensory placodes arise (adenohypophyseal, olfactory, lens, trigeminal, epibranchial, otic). Members of the Six family of transcription factors are major regulators of PPE specification, in partnership with cofactor proteins such as Eya. Six gene activity also maintains tissue boundaries between the PPE, neural crest, and epidermis by repressing genes that specify the fates of those adjacent ectodermally derived domains. As the embryo acquires anterior-posterior identity, the PPE becomes transcriptionally regionalized, and it subsequently becomes subdivided into specific placodes with distinct developmental fates in response to signaling from adjacent tissues. Each placode is characterized by a unique transcriptional program that leads to the differentiation of highly specialized cells, such as neurosecretory cells, sensory receptor cells, chemosensory neurons, peripheral glia, and supporting cells. In this review, we summarize the transcriptional and signaling factors that regulate key steps of placode development, influence subsequent sensory neuron specification, and discuss what is known about mutations in some of the essential PPE genes that underlie human congenital syndromes.

AP-2α is required after lens vesicle formation to maintain lens integrity

BACKGROUND

Transcription factors are critical in regulating lens development. The AP-2 family of transcription factors functions in differentiation, cell growth and apoptosis, and in lens and eye development. AP-2α, in particular, is important in early lens development, and when conditionally deleted at the placode stage defective separation of the lens vesicle from the surface ectoderm results. AP-2α's role during later stages of lens development is unknown. To address this, the MLR10-Cre transgene was used to delete AP-2α from the lens epithelium beginning at embryonic day (E) 10.5.

RESULTS

The loss of AP-2α after lens vesicle separation resulted in morphological defects beginning at E18.5. By P4, a small highly vacuolated lens with a multilayered epithelium was evident in the MLR10-AP-2α mutants. Epithelial cells appeared elongated and expressed fiber cell specific βB1 and γ-crystallins. Epithelial cell polarity and lens cell adhesion was disrupted and accompanied by the misexpression of ZO-1, N-Cadherin, and β-catenin. Cell death was observed in the mutant lens epithelium between postnatal day (P) 14 and P30, and correlated with altered arrangements of cells within the epithelium.

CONCLUSIONS

Our findings demonstrate that AP-2α continues to be required after lens vesicle separation to maintain a normal lens epithelial cell phenotype and overall lens integrity and to ensure correct fiber cell differentiation.

The lens equator: a platform for molecular machinery that regulates the switch from cell proliferation to differentiation in the vertebrate lens

The vertebrate lens is a transparent, spheroidal tissue, located in the anterior region of the eye that focuses visual images on the retina. During development, surface ectoderm associated with the neural retina invaginates to form the lens vesicle. Cells in the posterior half of the lens vesicle differentiate into primary lens fiber cells, which form the lens fiber core, while cells in the anterior half maintain a proliferative state as a monolayer lens epithelium. After formation of the primary fiber core, lens epithelial cells start to differentiate into lens fiber cells at the interface between the lens epithelium and the primary lens fiber core, which is called the equator. Differentiating lens fiber cells elongate and cover the old lens fiber core, resulting in growth of the lens during development. Thus, lens fiber differentiation is spatially regulated and the equator functions as a platform that regulates the switch from cell proliferation to cell differentiation. Since the 1970s, the mechanism underlying lens fiber cell differentiation has been intensively studied, and several regulatory factors that regulate lens fiber cell differentiation have been identified. In this review, we focus on the lens equator, where these regulatory factors crosstalk and cooperate to regulate lens fiber differentiation. Normally, lens epithelial cells must pass through the equator to start lens fiber differentiation. However, there are reports that when the lens epithelium structure is collapsed, lens fiber cell differentiation occurs without passing the equator. We also discuss a possible mechanism that represses lens fiber cell differentiation in lens epithelium.

Ectopic activation of Wnt/β-catenin signaling in lens fiber cells results in cataract formation and aberrant fiber cell differentiation

The Wnt/β-catenin signaling pathway controls many processes during development, including cell proliferation, cell differentiation and tissue homeostasis, and its aberrant regulation has been linked to various pathologies. In this study we investigated the effect of ectopic activation of Wnt/β-catenin signaling during lens fiber cell differentiation. To activate Wnt/β-catenin signaling in lens fiber cells, the transgenic mouse referred to as αA-CLEF was generated, in which the transactivation domain of β-catenin was fused to the DNA-binding protein LEF1, and expression of the transgene was controlled by αA-crystallin promoter. Constitutive activation of Wnt/β-catenin signaling in lens fiber cells of αA-CLEF mice resulted in abnormal and delayed fiber cell differentiation. Moreover, adult αA-CLEF mice developed cataract, microphthalmia and manifested downregulated levels of γ-crystallins in lenses. We provide evidence of aberrant expression of cell cycle regulators in embryonic lenses of αA-CLEF transgenic mice resulting in the delay in cell cycle exit and in the shift of fiber cell differentiation to the central fiber cell compartment. Our results indicate that precise regulation of the Wnt/β-catenin signaling activity during later stages of lens development is essential for proper lens fiber cell differentiation and lens transparency.

An allelic series at the paired box gene 6 (Pax6) locus reveals the functional specificity of Pax genes

The advent of the ocular and nervous system in metazoan evolution coincides with the diversification of a single ancestral paired box (Pax) gene into Pax6, Pax6(5a), and Pax2. To investigate the role of these Pax genes in neural development, we have generated an allelic series of knock-in models at the Pax6 locus. We showed that although Pax6(5a) and Pax2 could not replace Pax6 for its autoregulation in lens induction or for neural differentiation in retina, Pax6(5a) was sufficient for corneal-lenticular detachment. In brain development, cell proliferation in the cerebral cortex and dorsoventral patterning of the telencephalon and neural tube were partially rescued in either knock-in mutant. Contrary to the previous belief, our genetic studies showed that the Pax6 isoform Pax6(5a) could potentially play a role in neuronal differentiation in brain development. Importantly, Pax2 showed greater rescue efficiency than Pax6(5a) in the telencephalon even though the latter was identical to Pax6 outside the paired domain. In studying Ngn2, a Pax6 direct target gene in telencephalon, we showed that the level of Ngn2 expression correlated with the in vitro binding of Pax2, Pax6, and Pax6(5a) paired domain on its enhancer. Our results show that Pax6 is uniquely required for eye development, but in brain development, Pax6 can be functionally substituted by related Pax family genes that share a similar paired domain binding specificity.

Ocular surface development and gene expression

The ocular surface-a continuous epithelial surface with regional specializations including the surface and glandular epithelia of the cornea, conjunctiva, and lacrimal and meibomian glands connected by the overlying tear film-plays a central role in vision. Molecular and cellular events involved in embryonic development, postnatal maturation, and maintenance of the ocular surface are precisely regulated at the level of gene expression by a well-coordinated network of transcription factors. A thorough appreciation of the biological characteristics of the ocular surface in terms of its gene expression profiles and their regulation provides us with a valuable insight into the pathophysiology of various blinding disorders that disrupt the normal development, maturation, and/or maintenance of the ocular surface. This paper summarizes the current status of our knowledge related to the ocular surface development and gene expression and the contribution of different transcription factors to this process.

Patterns of gene expression in microarrays and expressed sequence tags from normal and cataractous lenses

In this contribution, we have examined the patterns of gene expression in normal and cataractous lenses as presented in five different papers using microarrays and expressed sequence tags. The purpose was to evaluate unique and common patterns of gene expression during development, aging and cataracts.

Pax6: a multi-level regulator of ocular development

Eye development has been a paradigm for the study of organogenesis, from the demonstration of lens induction through epithelial tissue morphogenesis, to neuronal specification and differentiation. The transcription factor Pax6 has been shown to play a key role in each of these processes. Pax6 is required for initiation of developmental pathways, patterning of epithelial tissues, activation of tissue-specific genes and interaction with other regulatory pathways. Herein we examine the data accumulated over the last few decades from extensive analyses of biochemical modules and genetic manipulation of the Pax6 gene. Specifically, we describe the regulation of Pax6's expression pattern, the protein's DNA-binding properties, and its specific roles and mechanisms of action at all stages of lens and retinal development. Pax6 functions at multiple levels to integrate extracellular information and execute cell-intrinsic differentiation programs that culminate in the specification and differentiation of a distinct ocular lineage.

Growth inhibition of human lens epithelial cells by short hairpin RNA in transcription factor forkhead box E3 (FOXE3)

BACKGROUND

Posterior capsule opacification occurs mainly due to the remnant lens epithelial cell proliferation and migration after cataract surgery. The purpose of this study was to investigate whether small hairpin RNA (shRNA)-mediated gene silencing of transcription factor forkhead box E3 (FOXE3) can be employed to inhibit the expression of FOXE3 and suppress the growth in lens epithelial cells.

METHODS

FOXE3-targeted shRNA was transfected into a human lens epithelial cell line (HLEB-3) using Lipofectamine 2000 reagent. Quantitative PCR was used to confirm the downregulation of FOXE3 mRNA expression following infection of lens epithelial cells, and FOXE3 protein expression levels were evaluated by Western blot analysis and immunofluorescence staining. HLEB-3 cell growth after the transduction was analyzed by cell counting and MTT colorimetric assay. Cell cycle of the HLEB-3 cells was examined by flowcytometric analysis.

RESULTS

Compared with the control groups, both mRNA and protein levels of FOXE3 expression were significantly decreased in shRNA-treated groups, and cytostatic effects were obvious within 48 h after transfection. An increased incidence of G1-phase arrest was identified in FOXE3-shRNA transfected HLEB-3 cells.

CONCLUSIONS

shRNA-mediated gene silencing of FOXE3 could significantly inhibit cell growth and induce the G1-phase arrest in HLEB-3 cells. Formation of posterior capsular opacification might be repressed if lens epithelial cell growth ceases after the FOXE3 gene is silenced with molecular biology technology.

Loss of Msx2 function down-regulates the FoxE3 expression and results in anterior segment dysgenesis resembling Peters anomaly

Complex molecular interactions dictate the developmental steps that lead to a mature and functional cornea and lens. Peters anomaly is one subtype of anterior segment dysgenesis especially due to abnormal development of the cornea and lens. MSX2 was recently implicated as a potential gene that is critical for anterior segment development. However, the role of MSX2 within the complex mechanisms of eye development remains elusive. Our present study observed the morphologic changes in conventional Msx2 knockout (KO) mice and found phenotypes consistent with Peters anomaly and microphthalmia seen in humans. The role of Msx2 in cornea and lens development was further investigated using IHC, in situ hybridization, and quantification of proliferative and apoptotic lens cells. Loss of Msx2 down-regulated FoxE3 expression and up-regulated Prox1 and crystallin expression in the lens. The FoxE3 and Prox1 malfunction and precocious Prox1 and crystallin expression contribute to a disturbed lens cell cycle in lens vesicles and eventually to cornea-lentoid adhesions and microphthalmia in Msx2 KO mice. The observed changes in the expression of FoxE3 suggest that Msx2 is an important contributor in controlling transcription of target genes critical for early eye development. These results provide the first direct genetic evidence of the involvement of MSX2 in Peters anomaly and the distinct function of MSX2 in regulating the growth and development of lens vesicles.

Transcription factors involved in lens development from the preplacodal ectoderm

Lens development is a stepwise process accompanied by the sequential activation of transcription factors. Transcription factor genes can be classified into three groups according to their functions: the first group comprises preplacodal genes, which are implicated in the formation of the preplacodal ectoderm that serves as a common primordium for cranial sensory tissues, including the lens. The second group comprises lens-specification genes, which establish the lens-field within the preplacodal ectoderm. The third group comprises lens-differentiation genes, which promote lens morphogenesis after the optic vesicle makes contact with the presumptive lens ectoderm. Analyses of the regulatory interactions between these genes have provided an overview of lens development, highlighting crucial roles for positive cross-regulation in fate specification and for feed-forward regulation in the execution of terminal differentiation. This overview also sheds light upon the mechanisms of how preplacodal gene activities lead to the activation of genes involved in lens-specification.

Conditional ablation of the Notch2 receptor in the ocular lens

Notch signaling is essential for proper lens development, however the specific requirements of individual Notch receptors have not been investigated. Here we report the lens phenotypes of Notch2 conditionally mutant mice, which exhibited severe microphthalmia, reduced pupillary openings, disrupted fiber cell morphology, eventual loss of the anterior epithelium, fiber cell dysgenesis, denucleation defects, and cataracts. Notch2 mutants also had persistent lens stalks as early as E11.5, and aberrant DNA synthesis in the fiber cell compartment by E14.5. Gene expression analyses showed that upon loss of Notch2, there were elevated levels of the cell cycle regulators Cdkn1a (p21Cip1), Ccnd2 (CyclinD2), and Trp63 (p63) that negatively regulates Wnt signaling, plus down-regulation of Cdh1 (E-Cadherin). Removal of Notch2 also resulted in an increased proportion of fiber cells, as was found in Rbpj and Jag1 conditional mutant lenses. However, Notch2 is not required for AEL proliferation, suggesting that a different receptor regulates this process. We found that Notch2 normally blocks lens progenitor cell death. Overall, we conclude that Notch2-mediated signaling regulates lens morphogenesis, apoptosis, cell cycle withdrawal, and secondary fiber cell differentiation.

A deletion in a cis element of Foxe3 causes cataracts and microphthalmia in rct mice

The Rinshoken cataract (rct) mutation, which causes congenital cataracts, is a recessive mutation found in SJL/J mice. All mutants present with opacity in the lens by 2 months of age. The rct locus was mapped to a 1.6-Mb region in Chr 4 that contains the Foxe3 gene. This gene is responsible for cataracts in humans and mice, and it plays a crucial role in the development of the lens. Furthermore, mutation of Foxe3 causes various ocular defects. We sequenced the genomic region of Foxe3, including the coding exons and UTRs; however, no mutations were discovered in these regions. Because there were no differences in Foxe3 sequences between the rct/rct and wild-type mice, we inferred that a mutation was located in the regulatory regions of the Foxe3 gene. To test this possibility, we sequenced a 5' noncoding region that is highly conserved among vertebrates and is predicted to be the major enhancer of Foxe3. This analysis revealed a deletion of 22-bp located approximately 3.2-kb upstream of the start codon of Foxe3 in rct mice. Moreover, we demonstrated by RT-PCR and in situ hybridization that the rct mutant has reduced expression of Foxe3 in the lens during development. We therefore suggest that cataracts in rct mice are caused by reduced Foxe3 expression in the lens and that this decreased expression is a result of a deletion in a cis-acting regulatory element.

The lens in focus: a comparison of lens development in Drosophila and vertebrates

The evolution of the eye has been a major subject of study dating back centuries. The advent of molecular genetics offered the surprising finding that morphologically distinct eyes rely on conserved regulatory gene networks for their formation. While many of these advances often stemmed from studies of the compound eye of the fruit fly, Drosophila melanogaster, and later translated to discoveries in vertebrate systems, studies on vertebrate lens development far outnumber those in Drosophila. This may be largely historical, since Spemann and Mangold's paradigm of tissue induction was discovered in the amphibian lens. Recent studies on lens development in Drosophila have begun to define molecular commonalities with the vertebrate lens. Here, we provide an overview of Drosophila lens development, discussing intrinsic and extrinsic factors controlling lens cell specification and differentiation. We then summarize key morphological and molecular events in vertebrate lens development, emphasizing regulatory factors and networks strongly associated with both systems. Finally, we provide a comparative analysis that highlights areas of research that would help further clarify the degree of conservation between the formation of dioptric systems in invertebrates and vertebrates.

Spry1 and Spry2 are necessary for lens vesicle separation and corneal differentiation

PURPOSE

The studies reported here were performed to analyze the roles of Sproutys (Sprys), downstream targets and negative feedback regulators of the fibroblast growth factor (FGF) signaling pathway, in lens and corneal differentiation.

METHODS

Spry1 and -2 were conditionally deleted in the lens and corneal epithelial precursors using the Le-Cre transgene and floxed alleles of Spry1 and -2. Alterations in lens and corneal development were assessed by hematoxylin and eosin staining, in situ hybridization, and immunohistochemistry.

RESULTS

Spry1 and -2 were upregulated in the lens fibers at the onset of fiber differentiation. FGF signaling was both necessary and sufficient for induction of Spry1 and -2 in the lens fiber cells. Spry1 and -2 single- or double-null lenses failed to separate from the overlying ectoderm and showed persistent keratolenticular stalks. Apoptosis of stalk cells, normally seen during lens vesicle detachment from the ectoderm, was inhibited in Spry mutant lenses, with concomitant ERK activation. Prox1 and p57(KIP2), normally upregulated at the onset of fiber differentiation were prematurely induced in the Spry mutant lens epithelial cells. However, terminal differentiation markers such as β- or γ-crystallin were not induced. Corneal epithelial precursors in Spry1 and -2 double mutants showed increased proliferation with elevated expression of Erm and DUSP6 and decreased expression of the corneal differentiation marker K12.

CONCLUSIONS

Collectively, the results indicate that Spry1 and -2 (1) through negative modulation of ERKs allow lens vesicle separation, (2) are targets of FGF signaling in the lens during initiation of fiber differentiation and (3) function redundantly in the corneal epithelial cells to suppress proliferation.

Building the developmental oculome: systems biology in vertebrate eye development and disease

The vertebrate eye is a sophisticated multicomponent organ that has been actively studied for over a century, resulting in the identification of the major embryonic and molecular events involved in its complex developmental program. Data gathered so far provides sufficient information to construct a rudimentary network of the various signaling molecules, transcription factors, and their targets for several key stages of this process. With the advent of genomic technologies, there has been a rapid expansion in our ability to collect and process biological information, and the use of systems-level approaches to study specific aspects of vertebrate eye development has already commenced. This is beginning to result in the definition of the dynamic developmental networks that operate in ocular tissues, and the interactions of such networks between coordinately developing ocular tissues. Such an integrative understanding of the eye by a comprehensive systems-level analysis can be termed the 'oculome', and that of serial developmental stages of the eye as it transits from its initiation to a fully formed functional organ represents the 'developmental oculome'. Construction of the developmental oculome will allow novel mechanistic insights that are essential for organ regeneration-based therapeutic applications, and the generation of computational models for eye disease states to predict the effects of drugs. This review discusses our present understanding of two of the individual components of the developing vertebrate eye--the lens and retina--at both the molecular and systems levels, and outlines the directions and tools required for construction of the developmental oculome.

The function of FGF signaling in the lens placode

Previous studies suggested that FGF signaling is important for lens formation. However, the times at which FGFs act to promote lens formation, the FGFs that are involved, the cells that secrete them and the mechanisms by which FGF signaling may promote lens formation are not known. We found that transcripts encoding several FGF ligands and the four classical FGF receptors are detectable in the lens-forming ectoderm at the time of lens induction. Conditional deletion of Fgfr1 and Fgfr2 from this tissue resulted in the formation of small lens rudiments that soon degenerated. Lens placodes lacking Fgfr1 and 2 were thinner than in wild-type embryos. Deletion of Fgfr2 increased cell death from the initiation of placode formation and concurrent deletion of Fgfr1 enhanced this phenotype. Fgfr1/2 conditional knockout placode cells expressed lower levels of proteins known to be regulated by FGF receptor signaling, but proteins known to be important for lens formation were present at normal levels in the remaining placode cells, including the transcription factors Pax6, Sox2 and FoxE3 and the lens-preferred protein αA-crystallin. Previous studies identified a genetic interaction between BMP and FGF signaling in lens formation and conditional deletion of Bmpr1a caused increased cell death in the lens placode, resulting in the formation of smaller lenses. In the present study, conditional deletion of both Bmpr1a and Fgfr2 increased cell death beyond that seen in Fgfr2(CKO) placodes and prevented lens formation. These results suggest that the primary role of autocrine or paracrine FGF signaling is to provide essential survival signals to lens placode cells. Because apoptosis was already increased at the onset of placode formation in Fgfr1/2 conditional knockout placode cells, FGF signaling was functionally absent during the period of lens induction by the optic vesicle. Since the expression of proteins required for lens formation was not altered in the knockout placode cells, we can conclude that FGF signaling from the optic vesicle is not required for lens induction.

Molecular mechanisms of pituitary organogenesis: In search of novel regulatory genes

Defects in pituitary gland organogenesis are sometimes associated with congenital anomalies that affect head development. Lesions in transcription factors and signaling pathways explain some of these developmental syndromes. Basic research studies, including the characterization of genetically engineered mice, provide a mechanistic framework for understanding how mutations create the clinical characteristics observed in patients. Defects in BMP, WNT, Notch, and FGF signaling pathways affect induction and growth of the pituitary primordium and other organ systems partly by altering the balance between signaling pathways. The PITX and LHX transcription factor families influence pituitary and head development and are clinically relevant. A few later-acting transcription factors have pituitary-specific effects, including PROP1, POU1F1 (PIT1), and TPIT (TBX19), while others, such as NeuroD1 and NR5A1 (SF1), are syndromic, influencing development of other endocrine organs. We conducted a survey of genes transcribed in developing mouse pituitary to find candidates for cases of pituitary hormone deficiency of unknown etiology. We identified numerous transcription factors that are members of gene families with roles in syndromic or non-syndromic pituitary hormone deficiency. This collection is a rich source for future basic and clinical studies.

Lens morphogenesis is dependent on Pax6-mediated inhibition of the canonical Wnt/beta-catenin signaling in the lens surface ectoderm

Lens formation in mouse is critically dependent on proper development of the retinal neuroectoderm that is located close beneath the head surface ectoderm. Signaling from the prospective retina triggers lens-specific gene expression in the surface-ectoderm. Supression of canonical Wnt/beta-catenin signaling in the surface ectoderm is one of the prerequisites for lens development because, as we show here, ectopic Wnt activation in the retina and lens abrogates lens formation. Wnt inhibiton is mediated by signals coming from the retina but its exact mechanism is unknown. We show that Pax6 directly controls expression of several Wnt inhibitors such as Sfrp1, Sfrp2, and Dkk1 in the presumptive lens. In accordance, absence of Pax6 function leads to aberrant canonical Wnt activity in the presumptive lens that subsequently impairs lens development. Thus Pax6 is required for down-regulation of canonical Wnt signaling in the presumptive lens ectoderm.

FOXE3 plays a significant role in autosomal recessive microphthalmia

FOXE3 forkhead transcription factor is essential to lens development in vertebrates. The eyes of Foxe3/foxe3-deficient mice and zebrafish fail to develop normally. In humans, autosomal dominant and recessive mutations in FOXE3 have been associated with variable phenotypes including anterior segment anomalies, cataract, and microphthalmia. We undertook sequencing of FOXE3 in 116 probands with a spectrum of ocular defects ranging from anterior segment dysgenesis and cataract to anophthalmia/microphthalmia. Recessive mutations in FOXE3 were found in four of 26 probands affected with bilateral microphthalmia (15% of all bilateral microphthalmia and 100% of consanguineous families with this phenotype). FOXE3-positive microphthalmia was accompanied by aphakia and/or corneal defects; no other associated systemic anomalies were observed in FOXE3-positive families. The previously reported c.720C > A (p.C240X) nonsense mutation was identified in two additional families in our sample and therefore appears to be recurrent, now reported in three independent microphthalmia families of varied ethnic backgrounds. Several missense variants were identified at varying frequencies in patient and control groups with some apparently being race-specific, which underscores the importance of utilizing race/ethnicity-matched control populations in evaluating the relevance of genetic screening results. In conclusion, FOXE3 mutations represent an important cause of nonsyndromic autosomal recessive bilateral microphthalmia.

Activated Ras alters lens and corneal development through induction of distinct downstream targets

Background

Mammalian Ras genes regulate diverse cellular processes including proliferation and differentiation and are frequently mutated in human cancers. Tumor development in response to Ras activation varies between different tissues and the molecular basis for these variations are poorly understood. The murine lens and cornea have a common embryonic origin and arise from adjacent regions of the surface ectoderm. Activation of the fibroblast growth factor (FGF) signaling pathway induces the corneal epithelial cells to proliferate and the lens epithelial cells to exit the cell cycle. The molecular mechanisms that regulate the differential responses of these two related tissues have not been defined. We have generated transgenic mice that express a constitutively active version of human H-Ras in their lenses and corneas.

Results

Ras transgenic lenses and corneal epithelial cells showed increased proliferation with concomitant increases in cyclin D1 and D2 expression. This initial increase in proliferation is sustained in the cornea but not in the lens epithelial cells. Coincidentally, cdk inhibitors p27^Kip1 and p57^Kip2 were upregulated in the Ras transgenic lenses but not in the corneas. Phospho-Erk1 and Erk2 levels were elevated in the lens but not in the cornea and Spry 1 and Spry 2, negative regulators of Ras-Raf-Erk signaling, were upregulated more in the corneal than in the lens epithelial cells. Both lens and corneal differentiation programs were sensitive to Ras activation. Ras transgenic embryos showed a distinctive alteration in the architecture of the lens pit. Ras activation, though sufficient for upregulation of Prox1, a transcription factor critical for cell cycle exit and initiation of fiber differentiation, is not sufficient for induction of terminal fiber differentiation. Expression of Keratin 12, a marker of corneal epithelial differentiation, was reduced in the Ras transgenic corneas.

Conclusions

Collectively, these results suggest that Ras activation a) induces distinct sets of downstream targets in the lens and cornea resulting in distinct cellular responses and b) is sufficient for initiation but not completion of lens fiber differentiation.

Seeing clearly: the dominant and recessive nature of FOXE3 in eye developmental anomalies

FOXE3 is a lens-specific transcription factor with a highly conserved forkhead domain previously implicated in congenital primary aphakia and anterior segment dysgenesis. Here, we identify new recessive FOXE3 mutations causative for microphthalmia, sclerocornea, primary aphakia, and glaucoma in two extended consanguineous families by SNP array genotyping followed by a candidate gene approach. Following an additional screen of 236 subjects with developmental eye anomalies, we report two further novel heterozygous mutations segregating in a dominant fashion in two different families. Although the dominant mutations were penetrant, they gave rise to highly variable phenotypes including iris and chorioretinal colobomas, Peters' anomaly, and isolated cataract (cerulean type and early onset adult nuclear and cortical cataract). Using in situ hybridization in human embryos, we demonstrate expression of FOXE3 restricted to lens tissue, predominantly in the anterior epithelium, suggesting that the extralenticular phenotypes caused by FOXE3 mutations are most likely to be secondary to abnormal lens formation. Our findings suggest that mutations in FOXE3 can give rise to a broad spectrum of eye anomalies, largely, but not exclusively related to lens development, and that both dominant and recessive inheritance patterns can be represented. We suggest including FOXE3 in the diagnostic genetic screening for these anomalies.

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

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

Pitx3 controls multiple aspects of lens development

The transcription factor Pitx3 is critical for lens formation. Deletions in the promoter of this gene cause abnormal lens development in the aphakia (ak) mouse mutant, which has only rudimentary lenses. In this study, we investigated the role of Pitx3 in lens development and differentiation. We found that reduced expression of Pitx3 leads to changes in the proliferation, differentiation and survival of lens cells. The genetic interactions between Pitx3 and Foxe3 were investigated, as these two transcription factors are expressed at the same time in lens development and their absence has similar consequences for lens development. We found no evidence that these two genes genetically interact. In general, our study shows that the abnormal phenotype of the ak lenses is not due to just one molecular pathway, rather in the absence of Pitx3 expression multiple aspects of lens development are disrupted.

Ocular forkhead transcription factors: seeing eye to eye

Forkhead transcription factors comprise a large family of proteins with diverse functions during development. Recently, there has been accumulating evidence that several members of this family of proteins play an important role in the development of the vertebrate retina. Here, we summarize the cumulative data which demonstrates the integral role that forkhead factors play in cell cycle control of retinal precursors, as well as in cell fate determination, during retinal development. The expression patterns for 14 retinal expressed forkhead transcription factors are presented with an emphasis on comparing the expression profiles across species. The functional data regarding forkhead gene products expressed within the retina are discussed. As presented, these data suggest that forkhead gene products contribute to the complex regulation of proliferation and differentiation of retinal precursors during vertebrate eye development.

Cell-autonomous requirement for rx function in the mammalian retina and posterior pituitary

Rx is a paired-like homeobox gene that is required for vertebrate eye formation. Mice lacking Rx function do not develop eyes or the posterior pituitary. To determine whether Rx is required cell autonomously in these tissues, we generated embryonic chimeras consisting of wild type and Rx−/− cells. We found that in the eye, Rx-deficient cells cannot participate in the formation of the neuroretina, retina pigment epithelium and the distal part of the optic stalk. In addition, in the ventral forebrain, Rx function is required cell autonomously for the formation of the posterior pituitary. Interestingly, Rx−/− and wild type cells segregate before the morphogenesis of these two tissues begins. Our observations suggest that Rx function is not only required for the morphogenesis of the retina and posterior pituitary, but also prior to morphogenesis, for the sorting out of cells to form distinct fields of retinal/pituitary cells.

Homeodomain protein Pitx3 maintains the mitotic activity of lens epithelial cells

Pitx3 is a bicoid like homeobox transcription factor of which deficiency in mice is linked with the aphakia phenotype. Mutation in human PITX3 gene is associated with autosomal dominant cataract with variable anterior segment mesenchymal dysgenesis. However, the molecular events causing the morphological changes in aphakia remains unknown. In this study we investigated the behaviour of GFP tagged Pitx3 null embryonic stem cells in chimeric lens, as well as the molecular features of the Pitx3-deficient lens of homozygous Pitx3 knockout mice. We show that the lack of colonisation of Pitx3-deficient ES cell derivatives in Pitx3 wild-type<-->Pitx3 null chimeric lens was due to the depletion of the epithelial cells in lens epithelium manifested by aberrant cell cycle exit and precocious onset of fibre cell differentiation of the Pitx3 null cells at the lens vesicle stage. This was demonstrated by the early activation of the cell cycle inhibitors p27Kip1 and p57Kip2, and the expression of beta-and gamma-crystallins. These defects are at least partially attributed to the loss of FoxE3 and misexpression of Prox1 in the lens vesicle epithelial cells. Thus, Pitx3 is essential to maintain lens epithelial phenotype and prevent inappropriate fibre cell differentiation during lens development.

Investigation of Frizzled-5 during embryonic neural development in mouse

Recent studies revealed that the Wnt receptor Frizzled-5 (Fzd5) is required for eye and retina development in zebrafish and Xenopus, however, its role during mammalian eye development is unknown. In the mouse embryo, Fzd5 is prominently expressed in the pituitary, distal optic vesicle, and optic stalk, then later in the progenitor zone of the developing retina. To elucidate the role of Fzd5 during eye development, we analyzed embryos with a germline disruption of the Fzd5 gene at E10.25, just before embryos die due to defects in yolk sac angiogenesis. We observed severe defects in optic cup morphogenesis and lens development. However, in embryos with conditional inactivation of Fzd5 using Six3-Cre, we observed no obvious early eye defects. Analysis of Axin2 mRNA expression and TCF/LEF-responsive reporter activation demonstrate that Fzd5 does not regulate the Wnt/beta-catenin pathway in the eye. Thus, the function of Fzd5 during eye development appears to be species-dependent.

Eye formation in the absence of retina

Eye development is a complex process that involves the formation of the retina and the lens, collectively called the eyeball, as well as the formation of auxiliary eye structures such as the eyelid, lacrimal gland, cornea and conjunctiva. The developmental requirements for the formation of each individual structure are only partially understood. We have shown previously that the homeobox-containing gene Rx is a key component in eye formation, as retinal structures do not develop and retina-specific gene expression is not observed in Rx-deficient mice. In addition, Rx-/- embryos do not develop any lens structure, despite the fact that Rx is not expressed in the lens. This demonstrates that during normal mammalian development, retina-specific gene expression is necessary for lens formation. In this paper we show that lens formation can be restored in Rx-deficient embryos experimentally, by the elimination of beta-catenin expression in the head surface ectoderm. This suggests that beta-catenin is involved in lens specification either through Wnt signaling or through its function in cell adhesion. In contrast to lens formation, we demonstrate that the development of auxiliary eye structures does not depend on retina-specific gene expression or retinal morphogenesis. These results point to the existence of two separate developmental processes involved in the formation of the eye and its associated structures. One involved in the formation of the eyeball and the second involved in the formation of the auxiliary eye structures.

Cell autonomous roles for AP-2alpha in lens vesicle separation and maintenance of the lens epithelial cell phenotype

In this study, we have created a conditional deletion of AP-2alpha in the developing mouse lens (Le-AP-2alpha mutants) to determine the cell-autonomous requirement(s) for AP-2alpha in lens development. Embryonic and adult Le-AP-2alpha mutants exhibited defects confined to lens placode derivatives, including a persistent adhesion of the lens to the overlying corneal epithelium (or lens stalk). Expression of known regulators of lens vesicle separation, including Pax6, Pitx3, and Foxe3 was observed in the Le-AP-2alpha mutant lens demonstrating that these genes do not lie directly downstream of AP-2alpha. Unlike germ-line mutants, Le-AP-2alpha mutants did not exhibit defects in the optic cup, further defining the tissue specific role(s) for AP-2alpha in eye development. Finally, comparative microarray analysis of lenses from the Le-AP-2alpha mutants vs. wild-type littermates revealed differential expression of 415 mRNAs, including reduced expression of genes important for maintaining the lens epithelial cell phenotype, such as E-cadherin.

Regulation and function of foxe3 during early zebrafish development

In this article, we investigate the expression, regulation, and function of the zebrafish forkhead gene foxe3. In wild type embryos, foxe3 is first expressed in a crescent-shaped area at the anterior end of the prechordal plate, corresponding to the polster. At later stages, the hatching gland, the lens, and the anterior pituitary express this gene. Using morpholinos against the zinc finger Kruppel-like factor 4 (KLF4) we show that foxe3 is regulated differently in the polster and in the lens. In the absence of KLF4, expression of foxe3 in the polster is not activated, whereas in the lens placode the expression of KLF4 is not required for the transcription of foxe3. The expression of foxe3 is also regulated by the hedgehog and nodal signaling pathways. foxe3 expression is altered in the hedgehog pathway mutants iguana and you-too and the nodal pathway mutant cyclops. foxe3 function is necessary for the execution of lens-specific gene expression and lens morphogenesis, as the knockdown of foxe3 results in a loss of platelet-derived growth factor receptor alpha (pdgfralpha) expression and in the vacuolization of the lens.

Anterior segment dysgenesis in the eyes of mice deficient for the high-mobility-group transcription factor Sox11

We describe that Sox11, a member of the group C of the Sox transcription factor family, is critically required during the morphogenetic processes of early eye development, and that lack of Sox11 results in ocular anterior segment dysgenesis (ASD). Sox11-deficient mice show a persistent lens stalk, a delay in lens formation, and the phenotypes of Peters' anomaly and microphthalmia at birth. In addition, the optic fissure does not close in the anterior halves of the eyes resulting in anterior coloboma. The delay in lens formation is associated with a reduced mitotic activity in the lens placode during its invagination into the optic cup. No changes in Pax6 expression are observed in the developing eyes of Sox11-/- mice, whereas the expression of Sox11 is reduced in optic cup, optic vesicle and lens placode of Pax6+/- embryos and in the optic vesicle of Pax6-/- mice. Transfection experiments show an increase in Sox11 expression when higher doses of Pax6 are present. Considerably smaller amounts of BMP7 are expressed in lens and optic cup of Sox11-/- mice as compared to their wild-type littermates. We conclude that Sox11 is required during separation of the lens vesicle from the surface ectoderm and the closure of the anterior optic fissure. The expression of Sox11 in early eye development is under control of Pax6, and changes in BMP7-signalling appear to be involved in the effects of Sox11 on anterior eye development. Our findings suggest that SOX11 might similarly be involved in the pathogenesis of ASD in human patients.

The orphan G protein-coupled receptor, Gpr161, encodes the vacuolated lens locus and controls neurulation and lens development

The vacuolated lens (vl) mouse mutant causes congenital cataracts and neural tube defects (NTDs), with the NTDs being caused by abnormal neural fold apposition and fusion. Our positional cloning of vl indicates these phenotypes result from a deletion mutation in an uncharacterized orphan G protein-coupled receptor (GPCR), Gpr161. Gpr161 displays restricted expression to the lateral neural folds, developing lens, retina, limb, and CNS. Characterization of the vl mutation indicates that C-terminal tail of Gpr161 is truncated, leading to multiple effects on the protein, including reduced receptor-mediated endocytosis. We have also mapped three modifier quantitative trait loci (QTL) that affect the incidence of either the vl cataract or NTD phenotypes. Bioinformatic, sequence, genetic, and functional data have determined that Foxe3, a key regulator of lens development, is a gene responsible for the vl cataract-modifying phenotype. These studies have extended our understanding of the vl locus in three significant ways. One, the cloning of the vl locus has identified a previously uncharacterized GPCR-ligand pathway necessary for neural fold fusion and lens development, providing insight into the molecular regulation of these developmental processes. Two, our QTL analysis has established vl as a mouse model for studying the multigenic basis of NTDs and cataracts. Three, we have identified Foxe3 as a genetic modifier that interacts with Gpr161 to regulate lens development.

Teleost lens development and degeneration

The transparent properties of the lens and its ability to focus light onto the retina are critical for normal vision. Optical clarity of the lens is achieved and maintained by a unique, highly regulated integration of lens cell proliferation and differentiation that persists throughout life. Zebrafish is a powerful genetic model for studying vertebrate lens differentiation and growth because the structural organization of the lens and gene functions are largely conserved with mammals, including humans. However, some features of zebrafish lens developmental morphology and gene expression are different from those of mammals and other terrestrial vertebrates. For example, the presumptive zebrafish lens delaminates from the surface ectoderm to form a solid mass of cells, in which the primary fibers differentiate by elongating in circular fashion. Both mutational and candidate gene analyses have identified and characterized developmental gene functions of the lens in zebrafish. This chapter presents the recent morphological analysis of zebrafish lens formation. In addition, the roles of Pitx3, Foxe3, and the lens-specific protein Lengsin (LENS Glutamine SYNthetase-like) in lens development are analyzed. Selected zebrafish lens mutants defective in early developmental processes and the maintenance of lens transparency are also discussed.

Convergence of a head-field selector Otx2 and Notch signaling: a mechanism for lens specification

Xenopus is ideal for systematic decoding of cis-regulatory networks because its evolutionary position among vertebrates allows one to combine comparative genomics with efficient transgenic technology in one system. Here, we have identified and analyzed the major enhancer of FoxE3 (Lens1), a gene essential for lens formation that is activated in the presumptive lens ectoderm (PLE) when commitment to the lens fate occurs. Deletion and mutation analyses of the enhancer based on comparison of Xenopus and mammalian sequences and in vitro and in vivo binding assays identified two essential transcriptional regulators: Otx2, a homeodomain protein expressed broadly in head ectoderm including the PLE, and Su(H), a nuclear signal transducer of Notch signaling. A Notch ligand, Delta2, is expressed in the optic vesicle adjacent to the PLE, and inhibition of its activity led to loss, or severe reduction, of FoxE3 expression followed by failure of placode formation. Ectopic activation of Notch signaling induced FoxE3 expression within head ectoderm expressing Otx2, and additional misexpression of Otx2 in trunk ectoderm extended the Notch-induced FoxE3 expression posteriorly. These data provide the first direct evidence of the involvement of Notch signaling in lens induction. The obligate integration of inputs of a field-selector (Otx2) and localized signaling (Notch) within target cis-regulatory elements might be a general mechanism of organ-field specification in vertebrates (as it is in Drosophila). This concept is also consistent with classical embryological studies of many organ systems involving a ;multiple-step induction'.