Abnormal migration and distribution of neural crest cells in Pax6 heterozygous mutant eye, a model for human eye diseases

Pax6 isoforms shape eye development: Insights from developmental stages and organoid models

Pax6 is a critical transcription factor involved in the development of the central nervous system. However, in humans, mutations in Pax6 predominantly result in iris deficiency rather than neurological phenotypes. This may be attributed to the distinct functions of Pax6 isoforms, Pax6a and Pax6b. In this study, we investigated the spatial and temporal expression patterns of Pax6 isoforms during different stages of mouse eye development. We observed a strong correlation between Pax6a expression and the neuroretina gene Sox2, while Pax6b showed a high correlation with iris-component genes, including the mesenchymal gene Foxc1. During early patterning from E10.5, Pax6b was expressed in the hinge of the optic cup and neighboring mesenchymal cells, whereas Pax6a was absent in these regions. At E14.5, both Pax6a and Pax6b were expressed in the future iris and ciliary body, coinciding with the integration of mesenchymal cells and Mitf-positive cells in the outer region. From E18.5, Pax6 isoforms exhibited distinct expression patterns as lineage genes became more restricted. To further validate these findings, we utilized ESC-derived eye organoids, which recapitulated the temporal and spatial expression patterns of lineage genes and Pax6 isoforms. Additionally, we found that the spatial expression patterns of Foxc1 and Mitf were impaired in Pax6b-mutant ESC-derived eye organoids. This in vitro eye organoids model suggested the involvement of Pax6b-positive local mesodermal cells in iris development. These results provide valuable insights into the regulatory roles of Pax6 isoforms during iris and neuroretina development and highlight the potential of ESC-derived eye organoids as a tool for studying normal and pathological eye development.

Congenital aniridia beyond black eyes: From phenotype and novel genetic mechanisms to innovative therapeutic approaches

Congenital PAX6-aniridia, initially characterized by the absence of the iris, has progressively been shown to be associated with other developmental ocular abnormalities and systemic features making congenital aniridia a complex syndromic disorder rather than a simple isolated disease of the iris. Moreover, foveal hypoplasia is now recognized as a more frequent feature than complete iris hypoplasia and a major visual prognosis determinant, reversing the classical clinical picture of this disease. Conversely, iris malformation is also a feature of various anterior segment dysgenesis disorders caused by PAX6-related developmental genes, adding a level of genetic complexity for accurate molecular diagnosis of aniridia. Therefore, the clinical recognition and differential genetic diagnosis of PAX6-related aniridia has been revealed to be much more challenging than initially thought, and still remains under-investigated. Here, we update specific clinical features of aniridia, with emphasis on their genotype correlations, as well as provide new knowledge regarding the PAX6 gene and its mutational spectrum, and highlight the beneficial utility of clinically implementing targeted Next-Generation Sequencing combined with Whole-Genome Sequencing to increase the genetic diagnostic yield of aniridia. We also present new molecular mechanisms underlying aniridia and aniridia-like phenotypes. Finally, we discuss the appropriate medical and surgical management of aniridic eyes, as well as innovative therapeutic options. Altogether, these combined clinical-genetic approaches will help to accelerate time to diagnosis, provide better determination of the disease prognosis and management, and confirm eligibility for future clinical trials or genetic-specific therapies.

Histone acetyltransferase KAT2A modulates neural stem cell differentiation and proliferation by inducing degradation of the transcription factor PAX6

Neural stem cells (NSCs) proliferation and differentiation rely on proper expression and post-translational modification of transcription factors involved in the determination of cell fate. Further characterization is needed to connect modifying enzymes with their transcription factor substrates in the regulation of these processes. Here, we demonstrated that the inhibition of KAT2A, a histone acetyltransferase, leads to a phenotype of small eyes in the developing embryo of zebrafish, which is associated with enhanced proliferation and apoptosis of NSCs in zebrafish eyes. We confirmed that this phenotype is mediated by the evaluated level of PAX6 protein. We further verified that KAT2A negatively regulates PAX6 at the protein level in cultured neural stem cells of rat cerebral cortex. We revealed that PAX6 is a novel acetylation substrate of KAT2A, and the acetylation of PAX6 promotes its ubiquitination mediated by the E3 ligase RNF8 that facilitated PAX6 degradation. Our study proposes that KAT2A inhibition results in accelerated proliferation, delayed differentiation, or apoptosis, depending on the context of PAX6 dosage. Thus, the KAT2A/PAX6 axis plays an essential role to keep a balance between the self-renewal and differentiation of NSCs.

Neural crest-derived cells possess differentiation potential to keratinocytes in the process of wound healing

Neural crest-derived cells (NCDCs), which exist as neural crest cells during the fetal stage and differentiate into palate cells, also exist in adult palate tissues, though with unknown roles. In the present study, NCDCs were labeled with EGFP derived from P0-Cre/CAG-CAT-EGFP (P0-EGFP) double transgenic mice, then their function in palate mucosa wound healing was analyzed. As a palate wound healing model, left-side palate mucosa of P0-EGFP mice was resected, and stem cell markers and keratinocyte markers were detected in healed areas. NCDCs were extracted from normal palate mucosa and precultured with stem cell media for 14 days, then were differentiated into keratinocytes or osteoblasts to analyze pluripotency. The wound healing process started with marginal mucosal regeneration on day two and the entire wound area was lined by regenerated mucosa with EGFP-positive cells (NCDCs) on day 28. EGFP-positive cells comprised approximately 60% of cells in healed oral mucosa, and 65% of those expressed stem cell markers (Sca-1⁺, PDGFRα⁺) and 30% expressed a keratinocyte marker (CK13⁺). In tests of cultured palate mucosa cells, approximately 70% of EGFP-positive cells expressed stem cell markers (Sca-1⁺, PDGFRα⁺). Furthermore, under differentiation inducing conditions, cultured EGFP-positive cells were successfully induced to differentiate into keratinocytes and osteoblasts. We concluded that NCDCs exist in adult palate tissues as stem cells and have potential to differentiate into various cell types during the wound healing process.

A transcriptome atlas of the mouse iris at single-cell resolution defines cell types and the genomic response to pupil dilation

The iris controls the level of retinal illumination by controlling pupil diameter. It is a site of diverse ophthalmologic diseases and it is a potential source of cells for ocular auto-transplantation. The present study provides foundational data on the mouse iris based on single nucleus RNA sequencing. More specifically, this work has (1) defined all of the major cell types in the mouse iris and ciliary body, (2) led to the discovery of two types of iris stromal cells and two types of iris sphincter cells, (3) revealed the differences in cell type-specific transcriptomes in the resting vs. dilated states, and (4) identified and validated antibody and in situ hybridization probes that can be used to visualize the major iris cell types. By immunostaining for specific iris cell types, we have observed and quantified distortions in nuclear morphology associated with iris dilation and clarified the neural crest contribution to the iris by showing that Wnt1-Cre-expressing progenitors contribute to nearly all iris cell types, whereas Sox10-Cre-expressing progenitors contribute only to stromal cells. This work should be useful as a point of reference for investigations of iris development, disease, and pharmacology, for the isolation and propagation of defined iris cell types, and for iris cell engineering and transplantation.

Pathophysiology of aniridia-associated keratopathy: Developmental aspects and unanswered questions

Aniridia, a rare congenital disease, is often characterized by a progressive, pronounced limbal insufficiency and ocular surface pathology termed aniridia-associated keratopathy (AAK). Due to the characteristics of AAK and its bilateral nature, clinical management is challenging and complicated by the multiple coexisting ocular and systemic morbidities in aniridia. Although it is primarily assumed that AAK originates from a congenital limbal stem cell deficiency, in recent years AAK and its pathogenesis has been questioned in the light of new evidence and a refined understanding of ocular development and the biology of limbal stem cells (LSCs) and their niche. Here, by consolidating and comparing the latest clinical and preclinical evidence, we discuss key unanswered questions regarding ocular developmental aspects crucial to AAK. We also highlight hypotheses on the potential role of LSCs and the ocular surface microenvironment in AAK. The insights thus gained lead to a greater appreciation for the role of developmental and cellular processes in the emergence of AAK. They also highlight areas for future research to enable a deeper understanding of aniridia, and thereby the potential to develop new treatments for this rare but blinding ocular surface disease.

Itpr1 regulates the formation of anterior eye segment tissues derived from neural crest cells

Mutations in ITPR1 cause ataxia and aniridia in individuals with Gillespie syndrome (GLSP). However, the pathogenic mechanisms underlying aniridia remain unclear. We identified a de novo GLSP mutation hotspot in the 3'-region of ITPR1 in five individuals with GLSP. Furthermore, RNA-sequencing and immunoblotting revealed an eye-specific transcript of Itpr1, encoding a 218amino acid isoform. This isoform is localized not only in the endoplasmic reticulum, but also in the nuclear and cytoplasmic membranes. Ocular-specific transcription was repressed by SOX9 and induced by MAF in the anterior eye segment (AES) tissues. Mice lacking seven base pairs of the last Itpr1 exon exhibited ataxia and aniridia, in which the iris lymphatic vessels, sphincter and dilator muscles, corneal endothelium and stroma were disrupted, but the neural crest cells persisted after completion of AES formation. Our analyses revealed that the 218-amino acid isoform regulated the directionality of actin fibers and the intensity of focal adhesion. The isoform might control the nuclear entry of transcriptional regulators, such as YAP. It is also possible that ITPR1 regulates both AES differentiation and muscle contraction in the iris.

Dysfunction of the limbal epithelial stem cell niche in aniridia-associated keratopathy

PURPOSE

Abnormalities in the limbal niche microenvironment have been suggested to be causally involved in aniridia-associated keratopathy (AAK), but histological analyses on the limbal structure and composition in AAK are lacking. Here, we investigated morphologic and molecular alterations of the limbal epithelial stem cell niche in human congenital aniridia.

METHODS

The blind, buphthalmic and painful left eye of a 16-year old girl with congenital aniridia and juvenile glaucoma had to be enucleated because of uncontrolled intraocular pressure. The diagnosis of AAK was based on classical clinical features and partial limbal stem cell deficiency in the superior half. Genetic analysis identified a large heterozygous PAX6 gene deletion encompassing exons 11-15 as well as exon 9 of the neighboring ELP4 gene. Three limbal biopsies were taken from the superior, nasal and temporal regions to isolate and cultivate limbal epithelial progenitor cells and subject them to mRNA expression analyses. The globe was vertically bisected and processed for light and transmission electron microscopy and immunohistochemistry.

RESULTS

Comparative analysis of the superior and inferior limbal zones showed a gradual degradation of palisade structures associated with the transition from a hyperplastic to an attenuated corneal epithelium, inflammatory cell infiltrations and basement membrane irregularities. The clinically unaffected inferior part revealed no distinct stem cell clusters in the preserved palisade region, but a uniform population of hyperproliferative, undifferentiated progenitor cells in the basal/suprabasal layers of limbal and corneal epithelia, which gave rise to maldifferentiated epithelial cells exhibiting a conjunctival/epidermal phenotype and nuclear-to-cytoplasmic translocation of Pax6. The structure of the limbal niche was fundamentally perturbed, showing marked alterations in extracellular matrix composition, dislocation of atypical melanocytes lacking melanosomes and melanin, aberrant Wnt/β-catenin and retinoic acid signaling, and massive immune cell infiltration.

CONCLUSIONS

Considering the limitations of a single Case study, the findings suggest that ocular surface alterations in AAK are caused by a primary dysfunction and gradual breakdown of the limbal stem cell niche through Pax6-related effects on both melanogenesis and epithelial differentiation.

Neural crest-derived cells in nasal conchae of adult mice contribute to bone regeneration

Neural crest-derived cells (NCDCs), a class of adult stem cells not restricted to embryonic tissues, are attractive tissue regenerative therapy candidates because of their ease of isolation, self-renewing properties, and multipotency. Although adult NCDCs can undergo osteogenic differentiation in vitro, whether they induce bone formation in vivo remains unclear. Previously, our group reported findings showing high amounts of NCDCs scattered throughout nasal concha tissues of adult mice. In the present study, NCDCs in nasal conchae labeled with enhanced green fluorescent protein (EGFP) were collected from adult P0-Cre/CAG-CAT-EGFP double transgenic mice, then cultured in serum-free medium to increase the number. Subsequently, NCDCs were harvested and suspended in type I atelocollagen gel, then an atelocollagen sponge was used as a scaffold for the cell suspension. Atelocollagen scaffolds with NCDCs were placed on bone defects created in a mouse calvarial bone defect model. Over the ensuing 12 weeks, micro-CT and histological analysis findings showed that mice with scaffolds containing NCDCs had slightly greater bone formation as compared to those with a scaffold alone. Furthermore, Raman spectroscopy revealed spectral properties of bone in mice that received scaffolds with NCDCs similar to those of native calvarial bone. Bone regeneration is important not only for gaining bone mass but also chemical properties. These results are the first to show the validity of biomolecule-free adult nasal concha-derived NCDCs for bone regeneration, including the chemical properties of regenerated bone tissue.

Transcriptomic analysis of differential gene expression during chick periocular neural crest differentiation into corneal cells

BACKGROUND

Multipotent neural crest cells (NCC) contribute to the corneal endothelium and keratocytes during ocular development, but the molecular mechanisms that underlie this process remain poorly understood. We performed RNA-Seq analysis on periocular neural crest (pNC), corneal endothelium, and keratocytes and validated expression of candidate genes by in situ hybridization.

RESULTS

RNA-Seq profiling revealed enrichment of genes between pNC and neural crest-derived corneal cells, which correspond to pathways involved in focal adhesion, ECM-receptor interaction, cell adhesion, melanogenesis, and MAPK signaling. Comparisons of candidate NCC genes to ocular gene expression revealed that majority of the NCC genes are expressed in the pNC, but they are either differentially expressed or maintained during corneal development. Several genes involved in retinoic acid, transforming growth factor-β, and Wnt signaling pathways and their modulators are also differentially expressed. We identified differentially expressed transcription factors as potential downstream candidates that may instruct expression of genes involved in establishing corneal endothelium and keratocyte identities.

CONCLUSION

Combined, our data reveal novel changes in gene expression profiles as pNC differentiate into highly specialized corneal endothelial cells and keratocytes. These data serve as platform for further analyses of the molecular networks involved in NCC differentiation into corneal cells and provide insights into genes involved in corneal dysgenesis and adult diseases.

Epistasis between Pax6Sey and genetic background reinforces the value of defined hybrid mouse models for therapeutic trials

The small eye (Sey) mouse is a model of PAX6-aniridia syndrome (aniridia). Aniridia, a congenital ocular disorder caused by heterozygous loss-of-function mutations in PAX6, needs new vision saving therapies. However, high phenotypic variability in Sey mice makes development of such therapies challenging. We hypothesize that genetic background is a major source of undesirable variability in Sey mice. Here we performed a systematic quantitative examination of anatomical, histological, and molecular phenotypes on the inbred C57BL/6J, hybrid B6129F1, and inbred 129S1/SvImJ backgrounds. The Sey allele significantly reduced eye weight, corneal thickness, PAX6 mRNA and protein levels, and elevated blood glucose levels. Surprisingly, Pax6^Sey/Sey brains had significantly elevated Pax6 transcripts compared to Pax6^+/+ embryos. Genetic background significantly influenced 12/24 measurements, with inbred strains introducing severe ocular and blood sugar phenotypes not observed in hybrid mice. Additionally, significant interactions (epistasis) between Pax6 genotype and genetic background were detected in measurements of eye weight, cornea epithelial thickness and cell count, retinal mRNA levels, and blood glucose levels. The number of epistatic interactions was reduced in hybrid mice. In conclusion, severe phenotypes in the unnatural inbred strains reinforce the value of more naturalistic F1 hybrid mice for the development of therapies for aniridia and other disorders.

Relationship between neural crest cell specification and rare ocular diseases

Development of the eye is closely associated with neural crest cell migration and specification. Eye development is extremely complex, as it requires the working of a combination of local factors, receptors, inductors, and signaling interactions between tissues such as the optic cup and periocular mesenchyme (POM). The POM is comprised of neural crest-derived mesenchymal progenitor cells that give rise to numerous important ocular structures including those tissues that form the optic cup and anterior segment of the eye. A number of genes are involved in the migration and specification of the POM such as PITX2, PITX3, FOXC1, FOXE3, PAX6, LMX1B, GPR48, TFAP2A, and TFAP2B. In this review, we will discuss the relevance of these genes in the development of the POM and how mutations and defects result in rare ocular diseases.

An analysis of anterior segment development in the chicken eye

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

Generating minicorneal organoids from human induced pluripotent stem cells

Corneal epithelial stem cells residing within the annular limbal crypts regulate adult tissue homeostasis. Autologous limbal grafts and tissue-engineered corneal epithelial cell sheets have been widely used in the treatment of various ocular surface defects. In the case of bilateral limbal defects, pluripotent stem cell (PSC)-derived corneal epithelial cells are now being explored as an alternative to allogeneic limbal grafts. Here, we report an efficient method to generate complex three-dimensional corneal organoids from human PSCs. The eye field primordial clusters that emerged from differentiating PSCs developed into whole eyeball-like, self-organized, three-dimensional, miniature structures consisting of retinal primordia, corneal primordia, a primitive eyelid-like outer covering and ciliary margin zone-like adnexal tissues in a stepwise maturation process within 15 weeks. These minicorneal organoids recapitulate the early developmental events in vitro and display similar anatomical features and marker expression profiles to adult corneal tissues. They offer an alternative tissue source for regenerating different layers of the cornea and eliminate the need for complicated cell enrichment procedures.

Induction of osteoblastic differentiation of neural crest-derived stem cells from hair follicles

The neural crest (NC) arises near the neural tube during embryo development. NC cells migrate throughout the embryo and have potential to differentiate into multiple cell types, such as peripheral nerves, glial, cardiac smooth muscle, endocrine, and pigment cells, and craniofacial bone. In the present study, we induced osteoblast-like cells using whisker follicles obtained from the NC of mice. Hair follicle cells derived from the NC labeled with enhanced green fluorescent protein (EGFP) were collected from protein zero-Cre/floxed-EGFP double transgenic mice and cultured, then treated and cultured in stem cell growth medium. After growth for 14 days, results of flow cytometry analysis showed that 95% of the EGFP-positive (EGFP+) hair follicle cells derived from the NC had proliferated and 76.2% of those expressed mesenchymal stem cells markers, such as platelet-derived growth factor α and stem cell antigen-1, and also showed constitutive expression of Runx2 mRNA. Cells stimulated with bone morphogenetic protein-2 expressed osteocalcin, osterix, and alkaline phosphatase mRNA, resulting in production of mineralized matrices, which were detected by von Kossa and alizarin red staining. Moreover, EGFP+ hair follicle cells consistently expressed macrophage colony-stimulating factor and osteoprotegerin (OPG). Addition of 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3] (10-8 M) to the cultures suppressed OPG expression and induced RANKL production in the cells. Furthermore, multinucleated osteoclasts appeared within 6 days after starting co-cultures of bone marrow cells with EGFP+ cells in the presence of 1,25(OH)2D3 and PGE2. These results suggest that NC-derived hair follicle cells possess a capacity for osteoblastic differentiation and may be useful for developing new bone regenerative medicine therapies.

Relationship between somatic mosaicism of Pax6 mutation and variable developmental eye abnormalities-an analysis of CRISPR genome-edited mouse embryos

The clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (Cas) system is a rapid gene-targeting technology that does not require embryonic stem cells. To demonstrate dosage effects of the Pax6 gene on eye formation, we generated Pax6-deficient mice with the CRISPR/Cas system. Eyes of founder embryos at embryonic day (E) 16.5 were examined and categorized according to macroscopic phenotype as class 1 (small eye with distinct pigmentation), class 2 (pigmentation without eye globes), or class 3 (no pigmentation and no eyes). Histologically, class 1 eyes were abnormally small in size with lens still attached to the cornea at E16.5. Class 2 eyes had no lens and distorted convoluted retinas. Class 3 eyes had only rudimentary optic vesicle-like tissues or histological anophthalmia. Genotyping of neck tissue cells from the founder embryos revealed somatic mosaicism and allelic complexity for Pax6. Relationships between eye phenotype and genotype were developed. The present results demonstrated that development of the lens from the surface ectoderm requires a higher gene dose of Pax6 than development of the retina from the optic vesicle. We further anticipate that mice with somatic mosaicism in a targeted gene generated by CRISPR/Cas-mediated genome editing will give some insights for understanding the complexity in human congenital diseases that occur in mosaic form.

Gene tracing analysis reveals the contribution of neural crest-derived cells in pituitary development

The anterior pituitary originates from the adenohypophyseal placode. Both the preplacode region and neural crest (NC) derive from subdivision of the neural border region, and further individualization of the placode domain is established by a reciprocal interaction between placodal precursors and NC cells (NCCs). It has long been known that NCCs are present in the adenohypophysis as interstitial cells. A recent report demonstrated that NCCs also contribute to the formation of pericytes in the developing pituitary. Here, we attempt to further clarify the role of NCCs in pituitary development using P0‐Cre/EGFP reporter mice. Spatiotemporal analyses revealed that GFP‐positive NCCs invaded the adenohypophysis in a stepwise manner. The first wave was detected on mouse embryonic day 9.5 (E9.5), when the pituitary primordium begins to be formed by adenohypophyseal placode cells; the second wave occurred on E14.5, when vasculogenesis proceeds from Atwell's recess. Finally, fate tracing of NCCs demonstrated that NC‐derived cells in the adenohypophysis terminally differentiate into all hormone‐producing cell lineages as well as pericytes. Our data suggest that NCCs contribute to pituitary organogenesis and vasculogenesis in conjunction with placode‐derived pituitary stem/progenitor cells.

Regional Volume Decreases in the Brain of Pax6 Heterozygous Mutant Rats: MRI Deformation-Based Morphometry

Pax6 is a transcription factor that pleiotropically regulates various developmental processes in the central nervous system. In a previous study, we revealed that Pax6 heterozygous mutant (rSey²/+) adult rats exhibit abnormalities in social interaction. However, the brain malformations underlying the behavioral abnormality are unknown. To elucidate the brain malformations in rSey²/+ rats, we morphometrically analyzed brains of rSey²/+ and wild type rats using small-animal magnetic resonance imaging (MRI). Sixty 10-week-old rats underwent brain MRI (29 rSey²/+ rats and 31 wild type rats). SPM8 software was used for image preprocessing and statistical image analysis. Normalized maps of the Jacobian determinant, a parameter for the expansion and/or contraction of brain regions, were obtained for each rat. rSey²/+ rats showed significant volume decreases in various brain regions including the neocortex, corpus callosum, olfactory structures, hippocampal formation, diencephalon, and midbrain compared to wild type rats. Among brain regions, the anterior commissure showed significant interaction between genotype and sex, indicating the effect of genotype difference on the anterior commissure volume was more robust in females than in males. The rSey²/+ rats exhibited decreased volume in various gray and white matter regions of the brain, which may contribute to manifestation of abnormal social behaviors.

Analysis of compound heterozygotes reveals that the mouse floxed Pax6 (tm1Ued) allele produces abnormal eye phenotypes

Analysis of abnormal phenotypes produced by different types of mutations has been crucial for our understanding of gene function. Some floxed alleles that retain a neomycin-resistance selection cassette (neo cassette) are not equivalent to wild-type alleles and provide useful experimental resources. Pax6 is an important developmental gene and the aim of this study was to determine whether the floxed Pax6^tm1Ued (Pax6^fl) allele, which has a retained neo cassette, produced any abnormal eye phenotypes that would imply that it differs from the wild-type allele. Homozygous Pax6^fl/fl and heterozygous Pax6^fl/+ mice had no overt qualitative eye abnormalities but morphometric analysis showed that Pax6^fl/fl corneas tended be thicker and smaller in diameter. To aid identification of weak effects, we produced compound heterozygotes with the Pax6^Sey-Neu (Pax6⁻) null allele. Pax6^fl/− compound heterozygotes had more severe eye abnormalities than Pax6^+/− heterozygotes, implying that Pax6^fl differs from the wild-type Pax6⁺ allele. Immunohistochemistry showed that the Pax6^fl/− corneal epithelium was positive for keratin 19 and negative for keratin 12, indicating that it was abnormally differentiated. This Pax6^fl allele provides a useful addition to the existing Pax6 allelic series and this study demonstrates the utility of using compound heterozygotes with null alleles to unmask cryptic effects of floxed alleles.

Identification of different mechanisms leading to PAX6 down-regulation as potential events contributing to the onset of Hirschsprung disease

Hirschsprung disease (HSCR) is attributed to a failure of neural crest derived cells to migrate, proliferate, differentiate or survive in the bowel wall during embryonic Enteric Nervous System (ENS) development. This process requires a wide and complex variety of molecules and signaling pathways which are activated by transcription factors. In an effort to better understand the etiology of HSCR, we have designed a study to identify new transcription factors participating in different stages of the colonization process. A differential expression study has been performed on a set of transcription factors using Neurosphere-like bodies from both HSCR and control patients. Differential expression levels were found for CDYL, MEIS1, STAT3 and PAX6. A significantly lower expression level for PAX6 in HSCR patients, would suit with the finding of an over-representation of the larger tandem (AC)m(AG)n repeats within the PAX6 promoter in HSCR patients, with the subsequent loss of protein P300 binding. Alternatively, PAX6 is a target for DNMT3B-dependant methylation, a process already proposed as a mechanism with a role in HSCR. Such decrease in PAX6 expression may influence in the proper function of signaling pathways involved in ENS with the confluence of additional genetic factors to the manifestation of HSCR phenotype.

Localization and osteoblastic differentiation potential of neural crest-derived cells in oral tissues of adult mice

In embryos, neural crest cells emerge from the dorsal region of the fusing neural tube and migrate throughout tissues to differentiate into various types of cells including osteoblasts. In adults, subsets of neural crest-derived cells (NCDCs) reside as stem cells and are considered to be useful cell sources for regenerative medicine strategies. Numerous studies have suggested that stem cells with a neural crest origin persist into adulthood, especially those within the mammalian craniofacial compartment. However, their distribution as well as capacity to differentiate into osteoblasts in adults is not fully understood. To analyze the precise distribution and characteristics of NCDCs in adult oral tissues, we utilized an established line of double transgenic (P0-Cre/CAG-CAT-EGFP) mice in which NCDCs express green fluorescent protein (GFP) throughout their life. GFP-positive cells were scattered like islands throughout tissues of the palate, gingiva, tongue, and buccal mucosa in adult mice, with those isolated from the latter shown to form spheres, typical cell clusters composed of stem cells, under low-adherent conditions. Furthermore, GFP-positive cells had markedly increased alkaline phosphatase (a marker enzyme of osteoblast differentiation) activity and mineralization as shown by alizarin red staining, in the presence of bone morphogenetic protein (BMP)-2. These results suggest that NCDCs reside in various adult oral tissues and possess potential to differentiate into osteoblastic cells. NCDCs in adults may be a useful cell source for bone regeneration strategies.

Identification of adult stem cells in Schwalbe's line region of the primate eye

PURPOSE

To identify stem cells in the chamber angle of the monkey eye by detection of 5-bromo-2'-deoxyuridine (BrdU) long-term retention.

METHODS

Four cynomolgus monkeys were treated with BrdU via subcutaneous pumps for 4 weeks. The eyes of two animals were processed immediately thereafter (group 1) while in the other animals, BrdU treatment was discontinued for 4 weeks to allow identification of cells with long-term BrdU retention (group 2). The number of BrdU-positive nuclei was quantified, and the cells were characterized by immunohistochemistry and transmission electron microscopy (TEM).

RESULTS

The number of BrdU-positive cells was higher at Schwalbe's line covering the peripheral end of Descemet's membrane than in Schlemm's canal (SC) endothelium, trabecular meshwork (TM), and scleral spur (SS). Labeling with BrdU in SC, TM, and SS was less intense and the number of labeled cells was smaller in group 2 than in group 1. In contrast, in cells of Schwalbe's line the intensity of BrdU staining and the number of BrdU-positive cells was similar when group 1 and 2 monkeys were compared with each other, indicating long-term BrdU retention. Cells that were BrdU-positive in Schwalbe's line region stained for the stem cell marker OCT4. Details of a stem cell niche in Schwalbe's line region were identified by TEM.

CONCLUSIONS

We provide evidence for a niche in the Schwalbe's line region harboring cells with long-term BrdU retention and OCT4 immunoreactivity. The cells likely constitute a population of adult stem cells with the capability to compensate for the loss of TM and/or corneal endothelial cells.

Hemizygous Le-Cre transgenic mice have severe eye abnormalities on some genetic backgrounds in the absence of LoxP sites

Eye phenotypes were investigated in Le-Cre^Tg/−; Pax6^fl/+ mice, which were expected to show tissue-specific reduction of Pax6 in surface ectoderm derivatives. To provide a better comparison with our previous studies of Pax6^+/− eye phenotypes, hemizygous Le-Cre^Tg/− and heterozygous Pax6^fl/+mice were crossed onto the CBA/Ca genetic background. After the Le-Cre transgene had been backcrossed to CBA/Ca for seven generations, significant eye abnormalities occurred in some hemizygous Le-Cre^Tg/−; Pax6^+/+ controls (without a floxed Pax6^fl allele) as well as experimental Le-Cre^Tg/−; Pax6^fl/+ mice. However, no abnormalities were seen in Le-Cre^−/−; Pax6^fl/+ or Le-Cre^−/−; Pax6^+/+ controls (without the Le-Cre transgene). The severity and frequency of the eye abnormalities in Le-Cre^Tg/−; Pax6^+/+ control mice diminished after backcrossing Le-Cre^Tg/− mice to the original FVB/N strain for two generations, showing that the effect was reversible. This genetic background effect suggests that the eye abnormalities are a consequence of an interaction between the Le-Cre transgene and alleles of unknown modifier genes present in certain genetic backgrounds. The abnormalities were also ameliorated by introducing additional Pax6 gene copies on a CBA/Ca background, suggesting involvement of Pax6 depletion in Le-Cre^Tg/−; Pax6^+/+ mice rather than direct action of Cre recombinase on cryptic pseudo-loxP sites. One possibility is that expression of Cre recombinase from the Pax6-Le regulatory sequences in the Le-Cre transgene depletes cofactors required for endogenous Pax6 gene expression. Our observation that eye abnormalities can occur in hemizygous Le-Cre^Tg/−; Pax6^+/+ mice, in the absence of a floxed allele, demonstrates the importance of including all the relevant genetic controls in Cre-loxP experiments.

The clinical characterization and surgical correction of blepharoptosis associated with congenital aniridia

PURPOSE

To document the clinical characteristics and describe our management of patients with blepharoptosis associated with congenital aniridia.

METHODS

Consecutive retrospective case series of patients with congenital aniridia seen at a single institution (Cincinnati Eye Institute) from 1963 to 2010. Surgical correction was performed by 2 surgeons (J.A.N. and R.C.K.).

RESULTS

Ptosis associated with congenital aniridia is marked with decreased levator function. Significant comorbid ophthalmologic disease was invariably present, particularly aniridia-associated keratopathy. Complications, most often keratopathy, were common, even following conservative correction.

CONCLUSIONS

The degree of ptosis is significant, and levator function is typically reduced. Ocular surface viability appears to play a key role in preoperative, intraoperative, and postoperative management. While we are aware that congenital aniridia is rather rare, we believe these recommendations are generalizable to patients with severe ocular surface disease.

Identification of gene expression profile of neural crest-derived cells isolated from submandibular glands of adult mice

Neural crest cells in the embryo migrate to reach target sites as neural crest-derived cells (NCDCs) where they differentiate into a variety of derivatives. Some NCDCs are maintained in an undifferentiated state throughout the life of the animal and are considered to be a useful cell source for regenerative medicine. However, no established method to obtain NCDCs sufficient for regenerative medicine from adults with high purity has been presented, since their distribution in adult tissues is not fully understood. It is critical to identify reliable markers for NCDCs in adults, as the expressions of P0 and Wnt1, the most reliable NCDC markers, are shut off in the embryonic stage. To analyze the characteristics of NCDCs in adult tissues, we utilized a double transgenic mouse strain, P0-Cre/CAG-CAT-EGFP transgenic mice (P0 mice), in which NCDCs were shown to express EGFP and we were able to recognize GFP-positive cells in those. We focused on the submandibular glands (SMGs), which are known to be derived from the neural crest. GFP-positive cells were shown to be scattered like islands in the SMGs of adult P0 mice. We surgically removed SMGs from adult mice and digested samples into single cell suspensions. GFP-positive cells separated using flow cytometry expressed a high level of Sox10, a marker of embryonic neural crest cells, suggesting successful isolation of NCDCs. To identify candidate marker genes in isolated NCDCs, we performed DNA microarray analyses and real-time PCR analysis of GFP-positive and -negative cells isolated from P0 mice, then selected genes showing differential gene expression patterns. As compared to GFP-negative cells, GFP-positive cells expressed Gpr4 and Ednrb at higher levels, whereas Pdgfra and Pdgfrb were expressed at lower levels. Furthermore, DNA microarray analysis showed that GFP-positive cells were positive for aquaporin 5, a marker for acinar cells. Together, our results indicate that NCDCs in adult SMGs have characteristic gene expression profiles specially their cell surface molecules. Cell sorting using a combination of these specific cell surface proteins would be a useful strategy for isolation of NCDCs from SMGs with high purity.

Molecular and cellular features of murine craniofacial and trunk neural crest cells as stem cell-like cells

The outstanding differentiation capacities and easier access from adult tissues, cells derived from neural crest cells (NCCs) have fascinated scientists in developmental biology and regenerative medicine. Differentiation potentials of NCCs are known to depend on their originating regions. Here, we report differential molecular features between craniofacial (cNCCs) and trunk (tNCCs) NCCs by analyzing transcription profiles and sphere forming assays of NCCs from P0-Cre/floxed-EGFP mouse embryos. We identified up-regulation of genes linked to carcinogenesis in cNCCs that were not previously reported to be related to NCCs, which was considered to be, an interesting feature in regard with carcinogenic potentials of NCCs such as melanoma and neuroblastoma. Wnt signal related genes were statistically up-regulated in cNCCs, also suggesting potential involvement of cNCCs in carcinogenesis. We also noticed intense expression of mesenchymal and neuronal markers in cNCCs and tNCCs, respectively. Consistent results were obtained from in vitro sphere-forming and differentiation assays. These results were in accordance with previous notion about differential potentials of cNCCs and tNCCs. We thus propose that sorting NCCs from P0-Cre/floxed-EGFP mice might be useful for the basic and translational research of NCCs. Furthermore, these newly-identified genes up-regulated in cNCC would provide helpful information on NC-originating tumors, developmental disorders in NCC derivatives, and potential applications of NCCs in regenerative medicine.

Neural stem and progenitor cells in health and disease

Neural stem/progenitor cells (NSPCs) have the potential to differentiate into neurons, astrocytes, and/or oligodendrocytes. Because these cells can be expanded in culture, they represent a vast source of neural cells. With the recent discovery that patient fibroblasts can be reprogrammed directly into induced NSPCs, the regulation of NSPC fate and function, in the context of cell-based disease models and patient-specific cell-replacement therapies, warrants review.

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.

Neural crest-derived horizontal basal cells as tissue stem cells in the adult olfactory epithelium

Horizontal basal cells (HBCs) have garnered attention as tissue stem cells of the olfactory epithelium (OE); however, these cells' exact lineage and their contributions to OE regeneration remain unknown. Neural crest-derived cells (NCDCs) have been shown to possess stem cell properties and to participate in the normal development of the OE. However, the contributions of NCDCs to both normal and regenerating adult OE remain unclear. In this study, we investigated the contribution of NCDCs to the OE, focusing particularly on HBCs. Using immunohistochemistry, we observed the OE of P0-Cre/EGFP mice expressing EGFP-tagged NCDCs at several stages of normal development along with regenerated OE following methimazole treatment. We observed EGFP expression in the HBCs, sustentacular cells (SUSs), Bowman's glands, olfactory receptor neurons (ORNs), and olfactory ensheathing cells of 6-week-old mice. No ectopic Cre expression was identified. Although HBCs at late embryonic stages were placode-derived (i.e., EGFP-negative), we found that EGFP+ HBCs alternatively increased with the decrease of placode-derived HBCs during maturation. In regenerated OE, the percentages of neural crest-derived ORNs and SUSs significantly increased compared with normal OE. These results suggest that NCDCs contribute greatly to the adult HBC population and that they are important for the maintenance of the OE.

Eye development and retinogenesis

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

Penetration and differentiation of cephalic neural crest-derived cells in the developing mouse telencephalon

Neural crest (NC) cells originate from the neural folds and migrate into the various embryonic regions where they differentiate into multiple cell types. A population of cephalic neural crest-derived cells (NCDCs) penetrates back into the developing forebrain to differentiate into microvascular pericytes, but little is known about when and how cephalic NCDCs invade the telencephalon and differentiate into pericytes. Using a transgenic mouse line in which NCDCs are genetically labeled with enhanced green fluorescent protein (EGFP), we observed that NCDCs started to invade the telencephalon together with endothelial cells from embryonic day (E) 9.5. A majority of NCDCs located in the telencephalon expressed pericyte markers, that is, PDGFRβ and NG2, and differentiated into pericytes around E11.5. Surprisingly, many of the NC-derived pericytes express p75, an undifferentiated NCDC marker at E11.5, as well as NCDCs in the mesenchyme. At the same time, a minor population of NCDCs that located separately from blood vessels in the telencephalon were NG2-negative and some of these NCDCs also expressed p75. Proliferation and differentiation of pericytes appeared to occur in a specific mesenchymal region where blood vessels penetrated into the telencephalon. These results indicate that (i) NCDCs penetrate back into the telencephalon in parallel with angiogenesis, (ii) many NC-derived pericytes may be still in pre-mature states even though after differentiation into pericytes in the early developing stages, (iii) a small minority of NCDCs may retain undifferentiated states in the developing telencephalon, and (iv) a majority of NCDCs proliferate and differentiate into pericytes in the mesenchyme around the telencephalon.

[Development of the human eye]

The development of the human eye requires a coordinated interplay between cells from different origins. The optic cup which is neuroectodermal in origin and derives from the neural tube, gives rise to the neuronal retina, the retinal pigmented epithelium, the epithelial layers of ciliary body and iris, and the iris musculature. The lens which is displaced into the optic cup during development originates from the surface ectoderm. Cells of the neural crest provide the ocular mesenchyme while ocular blood vessels are of mesodermal origin. The basic morphogenetic processes of eye development are completed at the end of the second month of embryonic life. However, for correct functioning further maturation processes are required which are not completed before birth or several months after. Examples are aqueous humor circulation, maturation of cones in the foveola, myelination of optic nerve axons and completion of the retinal vasculature.

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.

Effects of aberrant Pax6 gene dosage on mouse corneal pathophysiology and corneal epithelial homeostasis

Background

Altered dosage of the transcription factor PAX6 causes multiple human eye pathophysiologies. PAX6^+/− heterozygotes suffer from aniridia and aniridia-related keratopathy (ARK), a corneal deterioration that probably involves a limbal epithelial stem cell (LESC) deficiency. Heterozygous Pax6^+/Sey-Neu (Pax6^+/−) mice recapitulate the human disease and are a good model of ARK. Corneal pathologies also occur in other mouse Pax6 mutants and in PAX77^Tg/− transgenics, which over-express Pax6 and model human PAX6 duplication.

Methodology/Principal Findings

We used electron microscopy to investigate ocular defects in Pax6^+/− heterozygotes (low Pax6 levels) and PAX77^Tg/− transgenics (high Pax6 levels). As well as the well-documented epithelial defects, aberrant Pax6 dosage had profound effects on the corneal stroma and endothelium in both genotypes, including cellular vacuolation, similar to that reported for human macular corneal dystrophy. We used mosaic expression of an X-linked LacZ transgene in X-inactivation mosaic female (XLacZ^Tg/−) mice to investigate corneal epithelial maintenance by LESC clones in Pax6^+/− and PAX77^Tg/− mosaic mice. PAX77^Tg/− mosaics, over-expressing Pax6, produced normal corneal epithelial radial striped patterns (despite other corneal defects), suggesting that centripetal cell movement was unaffected. Moderately disrupted patterns in Pax6^+/− mosaics were corrected by introducing the PAX77 transgene (in Pax6^+/−, PAX77^Tg/− mosaics). Pax6^Leca4/+, XLacZ^Tg/− mosaic mice (heterozygous for the Pax6^Leca4 missense mutation) showed more severely disrupted mosaic patterns. Corrected corneal epithelial stripe numbers (an indirect estimate of active LESC clone numbers) declined with age (between 15 and 30 weeks) in wild-type XLacZ^Tg/− mosaics. In contrast, corrected stripe numbers were already low at 15 weeks in Pax6^+/− and PAX77^Tg/− mosaic corneas, suggesting Pax6 under- and over-expression both affect LESC clones.

Conclusions/Significance

Pax6^+/− and PAX77^Tg/− genotypes have only relatively minor effects on LESC clone numbers but cause more severe corneal endothelial and stromal defects. This should prompt further investigations of the pathophysiology underlying human aniridia and ARK.

Rare corneal clones in mice suggest an age-related decrease of stem cell activity and support the limbal epithelial stem cell hypothesis

The anterior ocular surface comprises the cornea, conjunctiva and a narrow intermediate region called the limbus. It is widely accepted that the corneal epithelium is maintained by stem cells but different hypotheses propose that the stem cells that maintain the mouse corneal epithelium during normal homeostasis are located either in the basal limbal epithelium or throughout the basal corneal epithelium. There are no specific markers to help test these alternatives and new methods are required to distinguish between them. We observed that KRT5(LacZ/-) transgenic mice produced rare β-galactosidase (β-gal)-positive radial stripes in the corneal epithelium. These stripes are likely to be clonal lineages of cells derived from stem cells, so they provide a lineage marker for actively proliferating stem cells. The distributions of the β-gal-positive radial stripes suggested they extended centripetally from the limbus, supporting the limbal epithelial stem cell (LESC) hypothesis. Stripe frequency declined between 15 and 30 weeks, which predicts a reduction in stem cell function with age. Pax6(+/-), KRT5(LacZ/-) corneas had small patches rather than stripes, which confirms that corneal maintenance is abnormal in Pax6(+/-) mice.

Pax6 regulates craniofacial form through its control of an essential cephalic ectodermal patterning center

Normal patterning and morphogenesis of the complex skeletal structures of the skull requires an exquisite, reciprocal cross-talk between the embryonic cephalic epithelia and mesenchyme. The mesenchyme associated with the jaws and the optic and olfactory capsules is derived from a Hox-negative cranial neural crest (CNC) population that acts much as an equivalence group in its interactions with specific local cephalic epithelial signals. Craniofacial pattern and morphogenesis is therefore controlled in large part through the regulation of these local cephalic epithelial signals. Here, we demonstrate that Pax6 is essential to the formation and maturation of the complex cephalic ectodermal patterning centers that govern the development and morphogenesis of the upper jaws and associated nasal capsules. Previous examinations of the craniofacial skeletal defects associated with Pax6 mutations have suggested that they arise from an optic-associated blockage in the migration of a specific subpopulation of midbrain CNC to the lateral frontonasal processes. We have addressed an alternative explanation for the craniofacial skeletal defects. We show that in Pax6(SeyN/SeyN) mutants regional CNC is present by E9.25 while there is already specific disruption in the early ontogenetic elaboration of cephalic ectodermal expression, associated with the nascent lambdoidal junction, of secreted signaling factors (including Fgf8 and Bmp4) and transcription factors (including Six1 and Dlx5) essential for upper jaw and/or nasal capsular development. Pax6 therefore regulates craniofacial form, at stages when CNC has just arrived in the frontonasal region, through its control of surface cephalic ectodermal competence to form an essential craniofacial patterning center.

Evaluation of Pax6 mutant rat as a model for autism

Autism is a highly variable brain developmental disorder and has a strong genetic basis. Pax6 is a pivotal player in brain development and maintenance. It is expressed in embryonic and adult neural stem cells, in astrocytes in the entire central nervous system, and in neurons in the olfactory bulb, amygdala, thalamus, and cerebellum, functioning in highly context-dependent manners. We have recently reported that Pax6 heterozygous mutant (rSey²/+) rats with a spontaneous mutation in the Pax6 gene, show impaired prepulse inhibition (PPI). In the present study, we further examined behaviors of rSey²/+ rats and revealed that they exhibited abnormality in social interaction (more aggression and withdrawal) in addition to impairment in rearing activity and in fear-conditioned memory. Ultrasonic vocalization (USV) in rSey²+ rat pups was normal in male but abnormal in female. Moreover, treatment with clozapine successfully recovered the defects in sensorimotor gating function, but not in fear-conditioned memory. Taken together with our prior human genetic data and results in other literatures, rSey²/+ rats likely have some phenotypic components of autism.

Excessive ingestion of long-chain polyunsaturated fatty acids during developmental stage causes strain- and sex-dependent eye abnormalities in mice

The eyes are rich in long-chain polyunsaturated fatty acids (LC-PUFAs) such as arachidonic acid [ARA; 20:4 (n-6)] and docosahexaenoic acid [DHA; 22:6 (n-3)]. Despite their abundance in the eyes, ARA and DHA cannot be sufficiently synthesized de novo in mammals. During gestation, eye development is exceptionally rapid, and substantial amounts of LC-PUFAs are needed to ensure proper eye development. Here, we studied the influences of dietary LC-PUFAs in dams (C57BL/6 and C3H/He) on the eye morphogenesis and organogenesis of their pups. Intriguingly, fetuses and newborn mice from C57BL/6 dams fed an LC-PUFA (particularly ARA)-enriched diet displayed a much higher incidence of eye abnormalities such as microphthalmia (small eye) and corneal opacity than those from dams fed an LC-PUFA-poor diet. The effects of LC-PUFAs on eye anomalies were evident only in the female pups of C57BL/6 inbred mice, not in those of C3H/He mice or male C57BL/6 mice. These results demonstrate a gene-by-environment (GxE) interaction in eye development in mice. Furthermore, our molecular analysis suggested the potential roles of Pitx3 and Pax6 in the above interaction involving ARA.

Reduced expression of Pax6 in lens and cornea of mutant mice leads to failure of chamber angle development and juvenile glaucoma

Heterozygous mutations in PAX6 are causative for aniridia, a condition that is frequently associated with juvenile glaucoma. Defects in morphogenesis of the iridocorneal angle, such as lack of trabecular meshwork differentiation, absence of Schlemm's canal and blockage of the angle by iris tissue, have been described as likely causes for glaucoma, and comparable defects have been observed in heterozygous Pax6-deficient mice. Here, we employed Cre/loxP-mediated inactivation of a single Pax6 allele in either the lens/cornea or the distal optic cup to dissect in which tissues both alleles of Pax6 need to be expressed to control the development of the tissues in the iridocorneal angle. Somatic inactivation of one allele of Pax6 exclusively from epithelial cells of lens and cornea resulted in the disruption of trabecular meshwork and Schlemm's canal development as well as in an adhesion between iris periphery and cornea in juvenile eyes, which resulted in the complete closure of the iridocorneal angle in the adult eye. Structural changes in the iridocorneal angle presumably caused a continuous increase in intraocular pressure leading to degenerative changes in optic nerve axons and to glaucoma. In contrast, the inactivation of a single Pax6 allele in the distal optic cup did not cause obvious changes in iridocorneal angle formation. We conclude that the defects in iridocorneal angle formation are caused by non-autonomous mechanisms due to Pax6 haploinsufficiency in lens or corneal epithelial cells. Pax6 probably controls the expression of signaling molecules in lens cells that regulate the morphogenetic processes during iridocorneal angle formation.

Semaphorin3A/neuropilin-1 signaling acts as a molecular switch regulating neural crest migration during cornea development

Cranial neural crest cells migrate into the periocular region and later contribute to various ocular tissues including the cornea, ciliary body and iris. After reaching the eye, they initially pause before migrating over the lens to form the cornea. Interestingly, removal of the lens leads to premature invasion and abnormal differentiation of the cornea. In exploring the molecular mechanisms underlying this effect, we find that semaphorin3A (Sema3A) is expressed in the lens placode and epithelium continuously throughout eye development. Interestingly, neuropilin-1 (Npn-1) is expressed by periocular neural crest but down-regulated, in a manner independent of the lens, by the subpopulation that migrates into the eye and gives rise to the cornea endothelium and stroma. In contrast, Npn-1 expressing neural crest cells remain in the periocular region and contribute to the anterior uvea and ocular blood vessels. Introduction of a peptide that inhibits Sema3A/Npn-1 signaling results in premature entry of neural crest cells over the lens that phenocopies lens ablation. Furthermore, Sema3A inhibits periocular neural crest migration in vitro. Taken together, our data reveal a novel and essential role of Sema3A/Npn-1 signaling in coordinating periocular neural crest migration that is vital for proper ocular development.

Effects of elevated Pax6 expression and genetic background on mouse eye development

PURPOSE

To analyze the effects of Pax6 overexpression and its interaction with genetic background on eye development.

METHODS

Histologic features of eyes from hemizygous PAX77(+/-) transgenic (high Pax6 gene dose) and wild-type mice were compared on different genetic backgrounds. Experimental PAX77(+/-)<-->wild-type and control wild-type<-->wild-type chimeras were analyzed to investigate the causes of abnormal eye development in PAX77(+/-) mice.

RESULTS

PAX77(+/-) mice showed an overlapping but distinct spectrum of eye abnormalities to Pax6(+/-) heterozygotes (low Pax6 dose). Some previously reported PAX77(+/-) eye abnormalities did not occur on all three genetic backgrounds examined. Several types of eye abnormalities occurred in the experimental PAX77(+/-)<-->wild-type chimeras, and they occurred more frequently in chimeras with higher contributions of PAX77(+/-) cells. Groups of RPE cells intruded into the optic nerve sheath, indicating that the boundary between the retina and optic nerve may be displaced. Both PAX77(+/-) and wild-type cells were involved in this ingression and in retinal folds, suggesting that neither effect was cell-autonomous. Cell-autonomous effects included failure of PAX77(+/-) and wild-type cells to mix normally and overrepresentation of PAX77(+/-) in the lens epithelium and RPE.

CONCLUSIONS

The extent of PAX77(+/-) eye abnormalities depended on PAX77(+/-) genotype, genetic background, and stochastic variation. Chimera analysis identified two types of cell-autonomous effects of the PAX77(+/-) genotype. Abnormal cell mixing between PAX77(+/-) and wild-type cells suggests altered expression of cell surface adhesion molecules. Some phenotypic differences between PAX77(+/-)<-->wild-type and Pax6(+/-)<-->wild-type chimeras may reflect differences in the levels of PAX77(+/-) and Pax6(+/-) contributions to chimeric lenses.

A nonautonomous role for retinal frizzled-5 in regulating hyaloid vitreous vasculature development

PURPOSE

Frizzled-5 (Fzd5) is expressed in the developing retina of multiple species and appears to play species-specific roles during eye development. The present study analyzed the effects of tissue-specific deletion of Fzd5 on mammalian eye development.

METHODS

To generate Fzd5 conditional knockout (CKO) mice, Fzd5(+/-) mice carrying the Six3-Cre transgene were crossed with Fzd5(LoxP/LoxP) mice. To determine which cell lineages in the eye had Cre recombinase activity, Six3-Cre transgenic mice were crossed with ROSA-26 reporter mice, and lacZ activity was assayed. Histologic analysis, immunofluorescence, and TUNEL labeling were performed from embryonic day (E)12.5 to postnatal stages to analyze vascularization, cell proliferation, retinal organization, and apoptosis.

RESULTS

On conditional disruption of Fzd5 specifically in the retina, but not in vitreous hyaloid vasculature (VHV), an abnormal accumulation consisting of pericytes and endothelial cells was observed in the vitreous as early as E12.5. The abundant retrolental cells persisted into postnatal stages and appeared as a pigmented intravitreal mass. In Fzd5 CKO mice there was failure of normal apoptosis of the VHV, and cells in the persistent VHV were maintained in the cell cycle up to postnatal day 23. Moreover, morphogenesis of the retina adjacent to the vasculature was disrupted, leading to retinal folds, detachment, and abnormal lamination. This phenotype is similar to that of human eye disease persistent hyperplastic primary vitreous (PHPV).

CONCLUSIONS

Selective loss of Fzd5 in the retina results in PHPV and retinal defects through an apparently cell-nonautonomous effect, revealing a potential requirement for retina-derived signals in regulating the development of the VHV.