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

Examination of Subbasal Nerve Plexus and Central Corneal Stromal Microstructure in Subjects With Congenital Aniridia, Using in Vivo Confocal Laser Scanning Microscopy

PURPOSE

During life up to 70% of aniridia subjects develop aniridia-associated keratopathy (AAK). AAK is characterized by limbal stem cell insufficiency, impaired corneal epithelial cell differentiation and abnormal cell adhesion, which leads to centripetal spreading vascularization, conjunctivalization, and thickening of the cornea. Our aim was to examine the subbasal nerve plexus and central corneal stromal microstructure in subjects with congenital aniridia, using in vivo confocal laser scanning microscopy CLSM.

METHODS

31 eyes of 18 patients (55.6% males, mean age: 25.22 ± 16.35 years) with congenital aniridia and 46 eyes of 29 healthy subjects (41.4% males, mean age 30 ± 14.82 years) were examined using the Rostock Cornea Module of Heidelberg Retina Tomograph-III. At the subbasal nerve plexus, corneal nerve fiber density (CNFD), corneal nerve fiber length (CNFL), corneal total branch density (CTBD), and corneal nerve fiber width (CNFW) were analyzed using ACCMetrics software. Keratocyte density in the anterior, middle and posterior stroma was assessed manually.

RESULTS

The CNFD (2.02 ± 4.08 vs 13.99 ± 6.34/mm2), CNFL (5.78 ± 2.68 vs 10.56 ± 2.82 mm/mm2) and CTBD (15.08 ± 15.62 vs 27.44 ± 15.05/mm2) were significantly lower in congenital aniridia subjects than in controls (p < 0.001 for all). CNFW was significantly higher in aniridia subjects than in controls (0.03 ± 0.004 vs 0.02 ± 0.003 mm/mm2) (p = 0.003). Keratocyte density was significantly lower in all stromal layers of aniridia subjects than in controls (p < 0.001 for all). Stromal alterations included confluent keratocytes, keratocytes with long extensions and hyperreflective dots between keratocytes in aniridia.

CONCLUSIONS

Decrease in CNFD, CNFL, and CTBD, as well as increase in CNFW well refer to the congenital aniridia-associated neuropathy. The decreased keratocyte density and the stromal alterations may be related to an increased cell death in congenital aniridia, nevertheless, stromal changes in different stages of AAK have to be further analyzed in detail.

Current Advances in Corneal Stromal Stem Cell Biology and Therapeutic Applications

Corneal stromal stem cells (CSSCs) are of particular interest in regenerative ophthalmology, offering a new therapeutic target for corneal injuries and diseases. This review provides a comprehensive examination of CSSCs, exploring their anatomy, functions, and role in maintaining corneal integrity. Molecular markers, wound healing mechanisms, and potential therapeutic applications are discussed. Global corneal blindness, especially in more resource-limited regions, underscores the need for innovative solutions. Challenges posed by corneal defects, emphasizing the urgent need for advanced therapeutic interventions, are discussed. The review places a spotlight on exosome therapy as a potential therapy. CSSC-derived exosomes exhibit significant potential for modulating inflammation, promoting tissue repair, and addressing corneal transparency. Additionally, the rejuvenation potential of CSSCs through epigenetic reprogramming adds to the evolving regenerative landscape. The imperative for clinical trials and human studies to seamlessly integrate these strategies into practice is emphasized. This points towards a future where CSSC-based therapies, particularly leveraging exosomes, play a central role in diversifying ophthalmic regenerative medicine.

New horizons in aniridia management: Clinical insights and therapeutic advances

Congenital aniridia is a rare genetic eye disorder characterized by the complete or partial absence of the iris from birth. Various theories and animal models have been proposed to understand and explain the pathogenesis of aniridia. In the majority of cases, aniridia is caused by a mutation in the PAX6 gene, which affects multiple structures within the eye. Treating these ocular complications is challenging and carries a high risk of side effects. However, emerging approaches for the treatment of aniridia-associated keratopathy, iris abnormalities, cataract abnormalities, and foveal hypoplasia show promise for improved outcomes. Genetic counseling plays a very important role to make informed choices. We also provide an overview of the newer diagnostic and therapeutic approaches such as next generation sequencing, gene therapy, in vivo silencing, and miRNA modulation.

rAAV-PHP.B escapes the mouse eye and causes lethality whereas rAAV9 can transduce aniridic corneal limbal stem cells without lethality

Recently safety concerns have been raised in connection with high doses of recombinant adeno-associated viruses (rAAV). Therefore, we undertook a series of experiments to test viral capsid (rAAV9 and rAAV-PHP.B), dose, and route of administration (intrastromal, intravitreal, and intravenous) focused on aniridia, a congenital blindness that currently has no cure. The success of gene therapy for aniridia may depend on the presence of functional limbal stem cells (LSCs) in the damaged aniridic corneas and whether rAAV can transduce them. Both these concerns were unknown, and thus were also addressed by our studies. For the first time, we report ataxia and lethality after intravitreal or intrastromal rAAV-PHP.B virus injections. We demonstrated virus escape from the eye and transduction of non-ocular tissues by rAAV9 and rAAV-PHP.B capsids. We have also shown that intrastromal and intravitreal delivery of rAAV9 can transduce functional LSCs, as well as all four PAX6-expressing retinal cell types in aniridic eye, respectively. Overall, lack of adverse events and successful transduction of LSCs and retinal cells makes it clear that rAAV9 is the capsid of choice for future aniridia gene therapy. Our finding of rAAV lethality after intraocular injections will be impactful for other researchers developing rAAV-based gene therapies.

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.

Corneal epithelial development and homeostasis

The corneal epithelium (CE), the most anterior cellular structure of the eye, is a self-renewing stratified squamous tissue that protects the rest of the eye from external elements. Each cell in this exquisite three-dimensional structure needs to have proper polarity and positional awareness for the CE to serve as a transparent, refractive, and protective tissue. Recent studies have begun to elucidate the molecular and cellular events involved in the embryonic development, post-natal maturation, and homeostasis of the CE, and how they are regulated by a well-coordinated network of transcription factors. This review summarizes the status of related knowledge and aims to provide insight into the pathophysiology of disorders caused by disruption of CE development, and/or homeostasis.

Swapping in lattice-based cell migration models

Cell migration is frequently modeled using on-lattice agent-based models (ABMs) that employ the excluded volume interaction. However, cells are also capable of exhibiting more complex cell-cell interactions, such as adhesion, repulsion, pulling, pushing, and swapping. Although the first four of these have already been incorporated into mathematical models for cell migration, swapping has not been well studied in this context. In this paper, we develop an ABM for cell movement in which an active agent can "swap" its position with another agent in its neighborhood with a given swapping probability. We consider a two-species system for which we derive the corresponding macroscopic model and compare it with the average behavior of the ABM. We see good agreement between the ABM and the macroscopic density. We also analyze the movement of agents at an individual level in the single-species as well as two-species scenarios to quantify the effects of swapping on an agent's motility.

PAX6 Expression Patterns in the Adult Human Limbal Stem Cell Niche

Paired box 6 (PAX6), a nuclear transcription factor, determines the fate of limbal epithelial progenitor cells (LEPC) and maintains epithelial cell identity. However, the expression of PAX6 in limbal niche cells, primarily mesenchymal stromal cells (LMSC), and melanocytes is scarce and not entirely clear. To distinctly assess the PAX6 expression in limbal niche cells, fresh and organ-cultured human corneoscleral tissues were stained immunohistochemically. Furthermore, the expression of PAX6 in cultured limbal cells was investigated. Immunostaining revealed the presence of PAX6-negative cells which were positive for vimentin and the melanocyte markers Melan-A and human melanoma black-45 in the basal layer of the limbal epithelium. PAX6 staining was not observed in the limbal stroma. Moreover, the expression of PAX6 was observed by Western blot in cultured LEPC but not in cultured LMSC or LM. These data indicate a restriction of PAX6 expression to limbal epithelial cells at the limbal stem cell niche. These observations warrant further studies for the presence of other PAX isoforms in the limbal stem cell niche.

Multiple roles of Pax6 in postnatal cornea development

Mammalian corneal development is a multistep process, including formation of the corneal epithelium (CE), endothelium and stroma during embryogenesis, followed by postnatal stratification of the epithelial layers and continuous renewal of the epithelium to replace the outermost corneal cells. Here, we employed the Cre-loxP system to conditionally deplete Pax6 proteins in two domains of ocular cells, i.e., the ocular surface epithelium (cornea, limbus and conjunctiva) (OSE) or postnatal CE via K14-cre or Aldh3-cre, respectively. Earlier and broader inactivation of Pax6 in the OSE resulted in thickened OSE with CE and limbal cells adopting the conjunctival keratin expression pattern. More restricted depletion of Pax6 in postnatal CE resulted in an abnormal cornea marked by reduced epithelial thickness despite increased epithelial cell proliferation. Immunofluorescence studies revealed loss of intermediate filament Cytokeratin 12 and diffused expression of adherens junction components, together with reduced tight junction protein, Zonula occludens-1. Furthermore, the expression of Cytokeratin 14, a basal cell marker in apical layers, indicates impaired differentiation of CE cells. Collectively, our data demonstrate that Pax6 is essential for maintaining proper differentiation and strong intercellular adhesion in postnatal CE cells, whereas limbal Pax6 is required to prevent the outgrowth of conjunctival cells to the cornea.

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.

Topical onion juice mitigates the morphological alterations of the cornea in the aged male rats

Aging is associated with structural and functional changes of the cornea. Fresh onion juice contains phenolic compounds and flavonoids that may provide an anti-aging effect. The aim of this study was to assess the ability of the onion juice to ameliorate these aging changes. Rats were grouped as adult and aged groups. Rats of both groups received eye drops of diluted onion juice in their right eyes every 8 hours for 12 weeks, while the left ones were served as control eyes. The corneas of both eyes underwent histopathological, immunohistochemical and morphometric assessments, in addition to measuring their intraocular pressure (IOP). The aged group exhibited a significantly elevated IOP, decreased tear secretion, degenerated corneal epithelium and endothelium, surface erosions and stromal edema with irregular collagen fibers. Administration of onion juice led to lowering of IOP, significant increase in tearing, restoration of most of epithelial, endothelial and stromal integrity, and increased epithelial, keratocystic and endothelial cell densities. Immunohistochemically, the epithelium and endothelium revealed positive immune reactions for both epidermal growth factor receptor (EGFR) and paired box protein-6 (PAX6) in the control and onion-treated corneas of the adult group, while these immune reactions were negative in the untreated aged ones. Onion drops in aged corneas showed a positive immune reaction for EFGR and PAX6 involving the epithelial and endothelial layers. In conclusion, topical onion juice improves corneal aging changes through its direct effect, and indirectly through lowering IOP and enhancing tear secretion.

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.

HC-HA/PTX3 from amniotic membrane reverts senescent limbal niche cells to Pax6+ neural crest progenitors to support limbal epithelial progenitors

Quiescence and self‐renewal of human corneal epithelial progenitor/stem cells (LEPC) are regulated by the limbal niche, presumably through close interaction with limbal (stromal) niche cells (LNC). Paired box homeotic gene 6 (Pax6), a conserved transcription factor essential for eye development, is essential for proper differentiation of limbal and corneal epithelial stem cells. Pax6 haploinsufficiency causes limbal stem cell deficiency, which leads to subsequent corneal blindness. We previously reported that serial passage of nuclear Pax6+ LNC resulted in the gradual loss of nuclear Pax6+ and neural crest progenitor status, the latter of which was reverted upon recovery of Pax6. These findings suggest Pax6 plays a pivotal role in supporting the self‐renewal of LEPC in limbal niche. Herein, we show that HC‐HA/PTX3, a unique matrix purified from amniotic membrane (AM) and consists of heavy chain 1of inter‐α‐trypsin inhibitor covalently linked to hyaluronic acid and complexed with pentraxin 3, is capable of reverting senescent LNC to nuclear Pax6+ neural crest progenitors that support self‐renewal of LEPC. Such reversion is causally linked to early cell aggregation mediated by activation of C‐X‐C chemokine receptor type 4 (CXCR4)‐mediated signaling followed by activation of bone morphogenetic protein (BMP) signaling. Furthermore, CXCR4‐mediated signaling, but not BMP signaling, controls recovery of the nuclear Pax6+ neural crest progenitors. These findings not only explain why AM helps in vivo and ex vivo expansion of human LEPC, but they also illuminate the potential role of HC‐HA/PTX3 as a surrogate matrix niche that complements stem cell‐based therapies in regenerative medicine.

Spatial correlations constrain cellular lifespan and pattern formation in corneal epithelium homeostasis

Homeostasis in adult tissues relies on the replication dynamics of stem cells, their progenitors and the spatial balance between them. This spatial and kinetic coordination is crucial to the successful maintenance of tissue size and its replenishment with new cells. However, our understanding of the role of cellular replicative lifespan and spatial correlation between cells in shaping tissue integrity is still lacking. We developed a mathematical model for the stochastic spatial dynamics that underlie the rejuvenation of corneal epithelium. Our model takes into account different spatial correlations between cell replication and cell removal. We derive the tradeoffs between replicative lifespan, spatial correlation length, and tissue rejuvenation dynamics. We determine the conditions that allow homeostasis and are consistent with biological timescales, pattern formation, and mutants phenotypes. Our results can be extended to any cellular system in which spatial homeostasis is maintained through cell replication.

PAX6, modified by SUMOylation, plays a protective role in corneal endothelial injury

Treating corneal endothelial diseases tends to be challenging as human corneal endothelial cells (CECs) do not proliferate in vivo. The pathogenesis or mechanisms underlying injured CECs need further studies. The abnormal expression of PAX6, which is an essential transcription factor for corneal homeostasis, exhibits corneal endothelial defects. However, the effects of PAX6 protein involved in corneal endothelial wound process are still unknown. Here, we found the upregulated protein levels of PAX6 in human corneal endothelial monolayer after injury; the expression of PAX6 also increased in murine and rat corneal endothelium injury models. Enforced PAX6 expression could alleviate the damages to CECs via regulating permeability by prompting cellular tight junction. In addition, SUMOylation mainly happened on both K53 and K89 residues of 48-kD PAX6 (the longest and main isoform expressed in cornea), and de-SUMOylation promoted the stability of PAX6 protein in vitro. In CECs of SENP1^+/− mice, increased SUMOylation levels leading to instability and low expression of PAX6, delayed the repair of CECs after injury. Furthermore, overexpression of PAX6 accelerated the rate of corneal endothelial repair of SENP1^+/− mice. Our findings indicate that SENP1-mediated de-SUMOylation improving the stability of PAX6, amplifies the protective effects of PAX6 on corneal endothelial injuries, highlighting potentials of PAX6 and/or SUMOylation to be used as a treatment target for corneal endothelial disorders.

A Systematic Review on Cornea Epithelial-Stromal Homeostasis

INTRODUCTION

This review aims to summarise the role of different cells, genes, proteins and lipid in regulating cornea epithelial-stromal homeostasis.

METHODS

We performed an Entrez PubMed literature search using keywords "human," "cornea," "epithelial," "stromal," "homeostasis," "fibrosis response," and "pathogenesis" on 24th of September 2019, resulting in 35 papers, of which 18 were chosen after filtering for "English language" and "published within 10 years" as well as curation for relevance by the authors.

RESULTS

The 18 selected papers showed that corneal epithelial cells, fibroblasts and telocytes, together with genes such as Klf4, Pax6 and Id found in the cells, play important roles in achieving homeostasis to maintain corneal integrity and transparency. Proteins classified as pro-fibrotic ligands and anti-fibrotic ligands are responsible for regulating cornea stromal fibrosis and extracellular matrix deposition, thus regulators of scar formation during wound healing. Anti-inflammatory ligands and wound repairing ligands are critical in eliciting protective inflammation and promoting epithelial healing, respectively. Protein receptors located on cellular membrane play a role in maintaining intercellular connections as well as corneal hydration.

DISCUSSION/CONCLUSION

These studies prompt development of novel therapeutic strategies such as tear drops or ointments that target certain proteins to maintain corneal homeostasis. However, more in vitro and in vivo studies are required to prove the effectiveness of exogenous administration of molecules in improving healing outcome. Hence, future investigations of the molecular pathways highlighted in this review will reveal novel therapeutic tools such as gene or cell therapy to treat corneal diseases.

Germline CRISPR/Cas9-Mediated Gene Editing Prevents Vision Loss in a Novel Mouse Model of Aniridia

Aniridia is a rare eye disorder, which is caused by mutations in the paired box 6 (PAX6) gene and results in vision loss due to the lack of a long-term vision-saving therapy. One potential approach to treating aniridia is targeted CRISPR-based genome editing. To enable the Pax6 small eye (Sey) mouse model of aniridia, which carries the same mutation found in patients, for preclinical testing of CRISPR-based therapeutic approaches, we endogenously tagged the Sey allele, allowing for the differential detection of protein from each allele. We optimized a correction strategy in vitro then tested it in vivo in the germline of our new mouse to validate the causality of the Sey mutation. The genomic manipulations were analyzed by PCR, as well as by Sanger and next-generation sequencing. The mice were studied by slit lamp imaging, immunohistochemistry, and western blot analyses. We successfully achieved both in vitro and in vivo germline correction of the Sey mutation, with the former resulting in an average 34.8% ± 4.6% SD correction, and the latter in restoration of 3xFLAG-tagged PAX6 expression and normal eyes. Hence, in this study we have created a novel mouse model for aniridia, demonstrated that germline correction of the Sey mutation alone rescues the mutant phenotype, and developed an allele-distinguishing CRISPR-based strategy for aniridia.

Pax 6 Controls Neural Crest Potential of Limbal Niche Cells to Support Self-Renewal of Limbal Epithelial Stem Cells

On ocular surface, corneal epithelial stem cells (SC) reside in limbus between cornea and conjunctiva. Pax6, an evolutionally conserved transcription factor essential for eye development, is expressed in post-natal corneal and limbal epithelia progenitors (LEPC) but not in underlying stroma. Because Pax6 is transiently expressed in developing corneal stroma and a subset of limbal and corneal stromal progenitors, we examined the role of Pax6 in limbal niche cells (LNC) in maintaining the phenotype of neural crest (NC) progenitors to support LEPC. Our results showed that nuclear Pax6 staining was found in freshly isolated LNC but not corneal stromal cells. Serial passaged LNC resulted in gradual loss of nuclear Pax6 (46 kDa) staining and neural crest progenitor status defined by the expression of embryonic SCs and NC markers, neurosphere formation, and differentiation into neurons, oligodendrocytes and astrocytes. Gain of function of 46 kDa Pax6 in late-passaged LNC resulted in nuclear Pax6 staining and promotion of the aforementioned NC progenitor status. In an in vitro reunion assay, early passaged LNC and late passaged LNC with overexpression of Pax6 inhibited the expression of corneal epithelial differentiation marker and promoted holoclone by LEPC. Therefore, expression of nuclear 46 kDa Pax6 in LNC plays an important developmental role in maintaining NC progenitor status to support self-renewal of corneal epithelial SCs in the limbal niche.

Computer simulation of neutral drift among limbal epithelial stem cells of mosaic mice

The use of mice that are mosaic for reporter gene expression underlies many lineage-tracing studies in stem cell biology. For example, using mosaic LacZ reporter mice, it was shown that limbal epithelial stem cells (LESCs) around the periphery of the cornea maintain radial sectors of the corneal epithelium and that radial stripe numbers declined with age. Originally, the corneal results were interpreted as progressive, age-related loss or irreversible inactivation of some LESC clones. In this study we used computer simulations to show that these results could also be explained by stochastic replacement of LESCs by neighbouring LESCs, leading to neutral drift of LESC populations. This was shown to reduce the number of coherent clones of LESCs and hence would coarsen the mosaic pattern in the corneal epithelium without reducing the absolute number of LESCs. Simulations also showed that corrected stripe numbers declined more slowly when LESCs were grouped non-randomly and that mosaicism was rarely lost unless simulated LESC numbers were unrealistically low. Possible reasons why age-related changes differ between mosaic corneal epithelia and other systems, such as adrenal cortices and intestinal crypts, are discussed.

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Age-related reduction of corneal stripes in mosaic mice was simulated.

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Stem cell loss and/or stem cell replacement reduced simulated stripe numbers.

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Stem cell replacement, without reducing stem cell numbers, caused neutral drift.

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Clumping of stem cells into larger groups caused slower decline in stripe numbers.

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Replacement rarely caused loss of mosaicism unless there were few stem cells.

Human aniridia limbal epithelial cells lack expression of keratins K3 and K12

Aniridia is a rare disease of the eye that affects the iris, lens and the cornea. In about 90% of the cases, patients showed a loss of PAX6 function. Patients with aniridia often develop aniridia-related keratopathy (ARK), due to limbal stem cell insufficiency. The aim of this study was to determine the differentiation status of limbal epithelial cells (LECs) in patients with ARK. Epithelial cells were isolated from the limbus region of two patients with aniridia and cultured in KSFM medium supplemented with EGF and BPE. Normal cells were obtained from limbus region of cadaveric control patients. Cells were analyzed with RT-PCR, qPCR and Western blot to evaluate expression of the developmental transcription factor, PAX6, potential stem cell markers, ΔNp63α and ABCG2, and corneal differentiation markers, keratin 12 (K12) and K3. Conjunctival differentiation markers, keratin 13 (K13) and K19 were also investigated. Cells were immunostained to evaluate K3, PAX6, and p63α protein expression. Protein coding sequence of PAX6 from patient LEC-cDNA was cloned and sequenced. RT-PCR showed that K3 and K12 transcripts were absent from patient cells, but present in healthy control preparations. Transcription levels of PAX6, ABCG2, and p63α of aniridia patients show no differences compared to normal control cells. Western blot showed reduced PAX6, protein levels in aniridia-LECs compared to control-LECs. Immunostaining also showed reduced PAX6 and K3 expression in aniridia-LECs compared to control-LECs. One aniridia patient showed a loss of stop codon in half of the cloned transcripts. In the second aniridia patient mRNA degradation through nonsense mediated decay seems to be very likely since we could not identify the mutation c.174C > T (Refseq. NM_000280), or misspliced transcripts in cDNA. We identified decreased PAX6 protein levels in aniridia patients in addition to decreased K12 mRNA levels compared to control cells. This result indicates an altered differentiation of limbal epithelial cells of aniridia patients. Further studies are necessary to evaluate the mechanism of differentiation of limbal epithelial cells in aniridia.

Characterization of slow cycling corneal limbal epithelial cells identifies putative stem cell markers

In order to identify reliable markers of corneal epithelial stem cells, we employed an inducible transgenic “pulse-chase” murine model (K5Tta × TRE-H2BGFP) to localize, purify, and characterize slow cycling cells in the cornea. The retention of GFP labeling in slowly dividing cells allowed for localization of these cells to the corneal limbus and their subsequent purification by FACS. Transcriptome analysis from slow cycling cells identified differentially expressed genes when comparing to GFP^- faster-dividing cells. RNA-Seq data from corneal epithelium were compared to epidermal hair follicle stem cell RNA-Seq to identify genes representing common putative stem cell markers or determinants, which included Sox9, Fzd7, Actn1, Anxa3 and Krt17. Overlapping retention of GFP and immunohistochemical expression of Krt15, ΔNp63, Sox9, Actn1, Fzd7 and Krt17 were observed in our transgenic model. Our analysis presents an array of novel genes as putative corneal stem cell markers.

Efficacy of Postnatal In Vivo Nonsense Suppression Therapy in a Pax6 Mouse Model of Aniridia

Nonsense mutations leading to premature stop codons are common occurring in approximately 12% of all human genetic diseases. Thus, pharmacological nonsense mutation suppression strategies would be beneficial to a large number of patients if the drugs could be targeted to the affected tissues at the appropriate time. Here, we used nonsense suppression to manipulate Pax6 dosage at different developmental times in the eye of the small eye (Pax6^Sey/+; G194X) mouse model of aniridia. Efficacy was assessed by functional assays for visual capacity, including electroretinography and optokinetic tracking (OKT), in addition to histological and biochemical studies. Malformation defects in the Pax6^Sey/+ postnatal eye responded to topically delivered nonsense suppression in a dose- and time-dependent manner. Elevated levels of Mmp9, a direct downstream target of Pax6 in the cornea, were observed with the different treatment regimens. The lens capsule was particularly sensitive to Pax6 dosage, revealing a potential new role for Pax6 in lens capsule maintenance and development. The remarkable capacity of malformed ocular tissue to respond postnatally to Pax6 dosage in vivo demonstrates that the use of nonsense suppression could be a valuable therapeutic approach for blinding diseases caused by nonsense mutations.

The core planar cell polarity gene, Vangl2, directs adult corneal epithelial cell alignment and migration

This study shows that the core planar cell polarity (PCP) genes direct the aligned cell migration in the adult corneal epithelium, a stratified squamous epithelium on the outer surface of the vertebrate eye. Expression of multiple core PCP genes was demonstrated in the adult corneal epithelium. PCP components were manipulated genetically and pharmacologically in human and mouse corneal epithelial cells in vivo and in vitro. Knockdown of VANGL2 reduced the directional component of migration of human corneal epithelial (HCE) cells without affecting speed. It was shown that signalling through PCP mediators, dishevelled, dishevelled-associated activator of morphogenesis and Rho-associated protein kinase directs the alignment of HCE cells by affecting cytoskeletal reorganization. Cells in which VANGL2 was disrupted tended to misalign on grooved surfaces and migrate across, rather than parallel to the grooves. Adult corneal epithelial cells in which Vangl2 had been conditionally deleted showed a reduced rate of wound-healing migration. Conditional deletion of Vangl2 in the mouse corneal epithelium ablated the normal highly stereotyped patterns of centripetal cell migration in vivo from the periphery (limbus) to the centre of the cornea. Corneal opacity owing to chronic wounding is a major cause of degenerative blindness across the world, and this study shows that Vangl2 activity is required for directional corneal epithelial migration.

Abnormal corneal epithelial maintenance in mice heterozygous for the micropinna microphthalmia mutation Mp

We investigated the corneal morphology of adult Mp/+ mice, which are heterozygous for the micropinna microphthalmia mutation, and identified several abnormalities, which implied that corneal epithelial maintenance was abnormal. The Mp/+ corneal epithelium was thin, loosely packed and contained goblet cells in older mice. Evidence also suggested that the barrier function was compromised. However, there was no major effect on corneal epithelial cell turnover and mosaic patterns of radial stripes indicated that radial cell movement was normal. Limbal blood vessels formed an abnormally wide limbal vasculature ring, K19-positive cells were distributed more widely than normal and K12 was weakly expressed in the peripheral cornea. This raises the possibilities that the limbal-corneal boundary was poorly defined or the limbus was wider than normal. BrdU label-retaining cell numbers and quantitative clonal analysis suggested that limbal epithelial stem cell numbers were not depleted and might be higher than normal. However, as corneal epithelial homeostasis was abnormal, it is possible that Mp/+ stem cell function was impaired. It has been shown recently that the Mp mutation involves a chromosome 18 inversion that disrupts the Fbn2 and Isoc1 genes and produces an abnormal, truncated fibrillin-2^MP protein. This abnormal protein accumulates in the endoplasmic reticulum (ER) of cells that normally express Fbn2 and causes ER stress. It was also shown that Fbn2 is expressed in the corneal stroma but not the corneal epithelium, suggesting that the presence of truncated fibrillin-2^MP protein in the corneal stroma disrupts corneal epithelial homeostasis in Mp/+ mice.

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Heterozygous mutant Mp/+ mice have small, abnormal eyes.

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The corneal epithelium is thin, loosely packed and has goblet cells.

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Corneal epithelial cell turnover and radial cell movement appear normal.

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The cornea-limbal border is poorly defined and the limbus appears wider than normal.

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Indirect tests suggest stem cells are not depleted and numbers might be increased.

Tracing the fate of limbal epithelial progenitor cells in the murine cornea

Stem cell (SC) division, deployment, and differentiation are processes that contribute to corneal epithelial renewal. Until now studying the destiny of these cells in a living mammal has not been possible. However, the advent of inducible multicolor genetic tagging and powerful imaging technologies has rendered this achievable in the translucent and readily accessible murine cornea. K14CreER(T2)-Confetti mice that harbor two copies of the Brainbow 2.1 cassette, yielding up to 10 colors from the stochastic recombination of fluorescent proteins, were used to monitor K-14(+) progenitor cell dynamics within the corneal epithelium in live animals. Multicolored columns of cells emerged from the basal limbal epithelium as they expanded and migrated linearly at a rate of 10.8 µm/day toward the central cornea. Moreover, the permanent expression of fluorophores, passed on from progenitor to progeny, assisted in discriminating individual clones as spectrally distinct streaks containing more than 1,000 cells within the illuminated area. The centripetal clonal expansion is suggestive that a single progenitor cell is responsible for maintaining a narrow corridor of corneal epithelial cells. Our data are in agreement with the limbus as the repository for SC as opposed to SC being distributed throughout the central cornea. This is the first report describing stem/progenitor cell fate determination in the murine cornea using multicolor genetic tracing. This model represents a powerful new resource to monitor SC kinetics and fate choice under homeostatic conditions, and may assist in assessing clonal evolution during corneal development, aging, wound-healing, disease, and following transplantation.

Fibroblast growth factor receptor 2 (FGFR2) is required for corneal epithelial cell proliferation and differentiation during embryonic development

Fibroblast growth factors (FGFs) play important roles in many aspects of embryonic development. During eye development, the lens and corneal epithelium are derived from the same surface ectodermal tissue. FGF receptor (FGFR)-signaling is essential for lens cell differentiation and survival, but its role in corneal development has not been fully investigated. In this study, we examined the corneal defects in Fgfr2 conditional knockout mice in which Cre expression is activated at lens induction stage by Pax6 P0 promoter. The cornea in LeCre, Fgfr2^loxP/loxP mice (referred as Fgfr2^CKO) was analyzed to assess changes in cell proliferation, differentiation and survival. We found that Fgfr2^CKO cornea was much thinner in epithelial and stromal layer when compared to WT cornea. At embryonic day 12.5–13.5 (E12.5–13.5) shortly after the lens vesicle detaches from the overlying surface ectoderm, cell proliferation (judged by labeling indices of Ki-67, BrdU and phospho-histone H3) was significantly reduced in corneal epithelium in Fgfr2^CKO mice. At later stage, cell differentiation markers for corneal epithelium and underlying stromal mesenchyme, keratin-12 and keratocan respectively, were not expressed in Fgfr2^CKO cornea. Furthermore, Pax6, a transcription factor essential for eye development, was not present in the Fgfr2^CKO mutant corneal epithelial at E16.5 but was expressed normally at E12.5, suggesting that FGFR2-signaling is required for maintaining Pax6 expression in this tissue. Interestingly, the role of FGFR2 in corneal epithelial development is independent of ERK1/2-signaling. In contrast to the lens, FGFR2 is not required for cell survival in cornea. This study demonstrates for the first time that FGFR2 plays an essential role in controlling cell proliferation and differentiation, and maintaining Pax6 levels in corneal epithelium via ERK-independent pathways during embryonic development.

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.

Increased corneal epithelial turnover contributes to abnormal homeostasis in the Pax6(+/-) mouse model of aniridia

We aimed to test previous predictions that limbal epithelial stem cells (LESCs) are quantitatively deficient or qualitatively defective in Pax6^+/− mice and decline with age in wild-type (WT) mice. Consistent with previous studies, corneal epithelial stripe patterns coarsened with age in WT mosaics. Mosaic patterns were also coarser in Pax6^+/− mosaics than WT at 15 weeks but not at 3 weeks, which excludes a developmental explanation and strengthens the prediction that Pax6^+/− mice have a LESC-deficiency. To investigate how Pax6 genotype and age affected corneal homeostasis, we compared corneal epithelial cell turnover and label-retaining cells (LRCs; putative LESCs) in Pax6^+/− and WT mice at 15 and 30 weeks. Limbal BrdU-LRC numbers were not reduced in the older WT mice, so this analysis failed to support the predicted age-related decline in slow-cycling LESC numbers in WT corneas. Similarly, limbal BrdU-LRC numbers were not reduced in Pax6^+/− heterozygotes but BrdU-LRCs were also present in Pax6^+/− corneas. It seems likely that Pax6^+/− LRCs are not exclusively stem cells and some may be terminally differentiated CD31-positive blood vessel cells, which invade the Pax6^+/− cornea. It was not, therefore, possible to use this approach to test the prediction that Pax6^+/− corneas had fewer LESCs than WT. However, short-term BrdU labelling showed that basal to suprabasal movement (leading to cell loss) occurred more rapidly in Pax6^+/− than WT mice. This implies that epithelial cell loss is higher in Pax6^+/− mice. If increased corneal epithelial cell loss exceeds the cell production capacity it could cause corneal homeostasis to become unstable, resulting in progressive corneal deterioration. Although it remains unclear whether Pax6^+/− mice have LESC-deficiency, we suggest that features of corneal deterioration, that are often taken as evidence of LESC-deficiency, might occur in the absence of stem cell deficiency if corneal homeostasis is destabilised by excessive cell loss.

Abnormal corneal endothelial maturation in collagen XII and XIV null mice

PURPOSE

Maturation of the endothelium and the adjacent matrix was characterized in wild-type (WT) mice. The influence of FACIT collagen XII and XIV deficiency on the morphology, maturation, and function of the corneal endothelium was examined.

METHODS

Analysis of the endothelium and Descemet's membrane (DM) was performed using transmission electron microscopy at postnatal day (P)4, P14, and P30 in WT, Col12a1(-/-), Col14a1(-/-), and Col12a1(-/-)/Col14a1(-/-) mice. Endothelial junctions were analyzed using ZO-1. The presence of endothelial-stromal communications was evaluated with phalloidin staining as well as electron microscopy. Finally, corneal thickness was assessed.

RESULTS

A thin DM, clefts between endothelial cells and DM, and large "vacuole-like" structures were present in the endothelial cells of WT mice at P4 but not noted at P30. The endothelia of Col12a1(-/-), Col14a1(-/-), and compound Col12a1(-/-)/Col14a1(-/-) in the P30 cornea maintained the vacuole-like structures seen at P4. A mature endothelial junction pattern was delayed in the null corneas. Expression of ZO-1 in WT endothelia at P14 was diffuse and localized to the basolateral and apical cell membrane. At P30, staining was localized to intercellular junctions. ZO-1 reactivity was patchy in Col12a1(-/-), Col14a1(-/-), and compound Col12a1(-/-)/Col14a1(-/-) corneas at P14 and P30. Stromal thickness was increased in P30 null corneas. Endothelial cell processes were demonstrated penetrating the DM and into the underlying stroma, throughout the entire endothelial layer in the P4 cornea.

CONCLUSIONS

Collagen XII and XIV null mice demonstrate delayed endothelial maturation. The structural alterations suggest functional changes in endothelial function resulting in increased corneal thickness. Endothelial-stromal interactions suggest a pathway for signal transduction.

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.

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.

Normal X-inactivation mosaicism in corneas of heterozygous FlnaDilp2/+ female mice--a model of human filamin A (FLNA) diseases

Background

Some abnormalities of mouse corneal epithelial maintenance can be identified by the atypical mosaic patterns they produce in X-chromosome inactivation mosaics and chimeras. Human FLNA/+ females, heterozygous for X-linked, filamin A gene (FLNA) mutations, display a range of disorders and X-inactivation mosaicism is sometimes quantitatively unbalanced. Flna^Dilp2/+ mice, heterozygous for an X-linked filamin A (Flna) nonsense mutation have variable eye, skeletal and other abnormalities, but X-inactivation mosaicism has not been investigated. The aim of this study was to determine whether X-inactivation mosaicism in the corneal epithelia of Flna^Dilp2/+ mice was affected in any way that might predict abnormal corneal epithelial maintenance.

Results

X-chromosome inactivation mosaicism was studied in the corneal epithelium and a control tissue (liver) of Flna^Dilp2/+ and wild-type (WT) female X-inactivation mosaics, hemizygous for the X-linked, LacZ reporter H253 transgene, using β-galactosidase histochemical staining. The corneal epithelia of Flna^Dilp2/+ and WT X-inactivation mosaics showed similar radial, striped patterns, implying epithelial cell movement was not disrupted in Flna^Dilp2/+ corneas. Corrected stripe numbers declined with age overall (but not significantly for either genotype individually), consistent with previous reports suggesting an age-related reduction in stem cell function. Corrected stripe numbers were not reduced in Flna^Dilp2/+ compared with WT X-inactivation mosaics and mosaicism was not significantly more unbalanced in the corneal epithelia or livers of Flna^Dilp2/+ than wild-type Flna^+/+ X-inactivation mosaics.

Conclusions

Mosaic analysis identified no major effect of the mouse Flna^Dilp2 mutation on corneal epithelial maintenance or the balance of X-inactivation mosaicism in the corneal epithelium or liver.

Interaction between hedgehog signalling and PAX6 dosage mediates maintenance and regeneration of the corneal epithelium

PURPOSE

To investigate the roles of intracellular signaling elicited by Hedgehog (Hh) ligands in corneal maintenance and wound healing.

METHODS

The expression of Hedgehog pathway components in the cornea was assayed by immunohistochemistry, western blot and reverse-transcription polymerase chain reaction (RT-PCR), in wild-type mice and mice that were heterozygous null for the gene encoding the transcription factor, paired box gene 6 (Pax6).  Corneal epithelial wound healing and cell migration assays were performed after pharmacological upregulation and downregulation of the hedgehog pathway.  Reporter mice, mosaic for expression of the gene encoding β-galactosidase (LacZ), were crossed to Pax6(+/-) mice, mice heterozygous for the gene encoding GLI-Kruppel family member GLI3, and Pax6(+/-)Gli3(+/-) double heterozygotes, to assay patterns of cell migration and corneal epithelial organization in vivo.

RESULTS

Corneal epithelial wound healing rates increased in response to application of Sonic hedgehog (Shh), but only in mice with wild-type Pax6 dosage.  Downregulation of Hedgehog signalling inhibited corneal epithelial cell proliferation.  Pax6(+/-) corneal epithelia showed increased proliferation in response to exogenous Shh, but not increased migration. Desert hedgehog (Dhh) was shown to be the major endogenous ligand, with Shh detectable only by RT-PCR and only after epithelial wounding. The activity of phosphatidylinositol-3-OH kinase-γ (PI3Kγ) was not required for the increased migration response in response to Shh.  Nuclear expression of the activator form of the transcription factor Gli3 (which mediates Hh signalling) was reduced in Pax6(+/-) corneal epithelia. Pax6(+/-)Gli3(+/-) double heterozygotes showed highly disrupted patterns of clonal arrangement of cells in the corneal epithelium.

CONCLUSIONS

The data show key roles for endogenous Dhh signalling in maintenance and regeneration of the corneal epithelium, demonstrate an interaction between Pax6 and Hh signalling in the corneal epithelium, and show that failure of Hh signalling pathways is a feature of Pax6(+/-) corneal disease that cannot be remedied pharmacologically by addition of the ligands.

Stem cells and corneal epithelial maintenance: insights from the mouse and other animal models

Maintenance of the corneal epithelium is essential for vision and is a dynamic process incorporating constant cell production, movement and loss. Although cell-based therapies involving the transplantation of putative stem cells are well advanced for the treatment of human corneal defects, the scientific understanding of these interventions is poor. No definitive marker that discriminates stem cells that maintain the corneal epithelium from the surrounding tissue has been discovered and the identity of these elusive cells is, therefore, hotly debated. The key elements of corneal epithelial maintenance have long been recognised but it is still not known how this dynamic balance is co-ordinated during normal homeostasis to ensure the corneal epithelium is maintained at a uniform thickness. Most indirect experimental evidence supports the limbal epithelial stem cell (LESC) hypothesis, which proposes that the adult corneal epithelium is maintained by stem cells located in the limbus at the corneal periphery. However, this has been challenged recently by the corneal epithelial stem cell (CESC) hypothesis, which proposes that during normal homeostasis the mouse corneal epithelium is maintained by stem cells located throughout the basal corneal epithelium with LESCs only contributing during wound healing. In this chapter we review experimental studies, mostly based on animal work, that provide insights into how stem cells maintain the normal corneal epithelium and consider the merits of the alternative LESC and CESC hypotheses. Finally, we highlight some recent research on other stem cell systems and consider how this could influence future research directions for identifying the stem cells that maintain the corneal epithelium.