Genetic aspects of embryonic eye development in vertebrates

MAP4K4 is involved in the neuronal development of retinal photoreceptors

Mitogen-activated protein kinase kinase kinase kinase-4 (MAP4K4) is a potential regulator of photoreceptor development. To investigate the mechanisms underlying MAP4K4 during the neuronal development of retinal photoreceptors, we generated knockout models of C57BL/6j mice in vivo and 661 W cells in vitro. Our findings revealed homozygous lethality and neural tube malformation in mice subjected to Map4k4 DNA ablation, providing evidence for the involvement of MAP4K4 in early stage embryonic neural formation. Furthermore, our study demonstrated that the ablation of Map4k4 DNA led to the vulnerability of photoreceptor neurites during induced neuronal development. By monitoring transcriptional and protein variations in mitogen-activated protein kinase (MAPK) signaling pathway-related factors, we discovered an imbalance in neurogenesis-related factors in Map4k4 -/- cells. Specifically, MAP4K4 promotes jun proto-oncogene (c-JUN) phosphorylation and recruits other factors related to nerve growth, ultimately leading to the robust formation of photoreceptor neurites. These data suggest that MAP4K4 plays a decisive role in regulating the fate of retinal photoreceptors through molecular modulation and contributes to our understanding of vision formation.

Disruption of Fetal Eye Development Caused by Insulin-induced Maternal Hypoglycemia in Rats

We previously revealed that insulin-induced severe and long-lasting maternal hypoglycemia in rats caused anophthalmia and microphthalmia in fetuses; however, it remained unclear whether hypoglycemia-induced eye anomalies were developmental retardation or disruption, and when and how they developed. Hence, we induced hypoglycemia in pregnant Sprague-Dawley rats by injecting insulin from Days 6 to 11 of pregnancy and performed periodical histopathological examination of fetal eyes from embryonic days (E)10 to 20. On E10, optic vesicle had developed normally both in the control and insulin-treated group; however, on E11, optic cup (OC) had developed in the control group but not in the insulin-treated group. On E12, neural retina (NR), retinal pigmented epithelium (RPE), lens, and presumptive cornea had been observed in the control group. In contrast, lens pit and OC with remaining space between RPE and NR had developed in the insulin-treated group. From E13 to E15, developmental disruption characterized by defects, hypoplasia, and degeneration in the retina, lens, and cornea was observed in the insulin-treated group, resulting in anophthalmia or microphthalmia on E20. Moreover, the expression of MITF and chx10, which are essential for early eye development by expressing in the presumptive retina and lens and regulating each other's expression level, was ectopic and suppressed on E11. In conclusion, insulin-induced maternal hypoglycemia caused developmental disruption, but not simple developmental retardation of fetal eyes, and its trigger might be a failure of presumptive retina and lens to interact on E11.

Outcomes of Human Leukocyte Antigen-Matched Allogeneic Cultivated Limbal Epithelial Transplantation in Aniridia-Associated Keratopathy-A Single-Center Retrospective Analysis

PURPOSE

To assess the efficacy and safety of human leukocyte antigen-matched allogeneic cultivated limbal epithelial stem cell grafts in the treatment of aniridia-associated keratopathy (AAK).

METHODS

Six eyes of 6 patients with severe AAK received an allogeneic stem cell graft between January 2010 and March 2017. Anatomical and functional results were assessed at 6 months, 1 year, 2 years, and the final follow-up visit available. Safety analysis was performed by considering all perioperative and postoperative adverse events and additional surgeries required during the follow-up period.

RESULTS

The mean follow-up was 53.6 months (range 24-104 months). In most patients (80%), there was an early improvement of the keratopathy postoperatively, which slowly regressed during longer follow-up. At the final follow-up, 4 of the eyes were graded as failure and 1 eye was graded as partial success. Grading the sixth eye was not possible because of an adverse event. None of the patients maintained a total anatomical success in the long-term. Only 1 patient maintained a modest improvement in best-corrected visual acuity from hand motion to counting fingers. Four serious adverse events were recorded in 2 patients.

CONCLUSIONS

Severe AAK remains a challenging condition to manage. Transplantation of allogenic ex vivo cultivated limbal stem cells may provide a temporary improvement in ocular surface stability, but anatomical and functional results are poor in the long-term. The eyes are prone to adverse events, and any surgical treatment should take this into consideration.

Discovery of a body-wide photosensory array that matures in an adult-like animal and mediates eye-brain-independent movement and arousal

The ability to respond to light has profoundly shaped life. Animals with eyes overwhelmingly rely on their visual circuits for mediating light-induced coordinated movements. Building on previously reported behaviors, we report the discovery of an organized, eye-independent (extraocular), body-wide photosensory framework that allows even a head-removed animal to move like an intact animal. Despite possessing sensitive cerebral eyes and a centralized brain that controls most behaviors, head-removed planarians show acute, coordinated ultraviolet-A (UV-A) aversive phototaxis. We find this eye-brain-independent phototaxis is mediated by two noncanonical rhabdomeric opsins, the first known function for this newly classified opsin-clade. We uncover a unique array of dual-opsin-expressing photoreceptor cells that line the periphery of animal body, are proximal to a body-wide nerve net, and mediate UV-A phototaxis by engaging multiple modes of locomotion. Unlike embryonically developing cerebral eyes that are functional when animals hatch, the body-wide photosensory array matures postembryonically in "adult-like animals." Notably, apart from head-removed phototaxis, the body-wide, extraocular sensory organization also impacts physiology of intact animals. Low-dose UV-A, but not visible light (ocular-stimulus), is able to arouse intact worms that have naturally cycled to an inactive/rest-like state. This wavelength selective, low-light arousal of resting animals is noncanonical-opsin dependent but eye independent. Our discovery of an autonomous, multifunctional, late-maturing, organized body-wide photosensory system establishes a paradigm in sensory biology and evolution of light sensing.

Gestational folic acid deficiency alters embryonic eye development: Possible role of basement membrane proteins in eye malformations

OBJECTIVES

Folic acid (FA) is crucial before and during early pregnancy. FA deficiency can occur because dietary FA intake is low in mothers at the time of conception. Likewise, various ocular pathologies are related to the alteration of extracellular matrices. The present study aimed to investigate the association between maternal FA deficiency and congenital eye defects. We also investigated whether maternal diet deficient in FA alters the expression of collagen IV and laminin-1 as a possible mechanism responsible for the appearance of ocular malformations. Both proteins are the main components of the basal lamina, and form an interlaced network that creates a relevant scaffold basement membrane. Basal laminae are involved in tissues maintenance and implicated in regulating many cellular processes.

METHODS

A total of 57 mouse embryos were classified into the following groups: Control group, (mothers were fed a standard rodent diet), and D2 and D8 groups (mothers were fed FA-deficient [FAD] diet for 2 or 8 wk, respectively). Female mice from group D2 were fed a FAD diet (0 mg/kg diet + 1% succinyl sulfathiazole used to block the synthesis of FA) for 2 wk from the day after mating until day 14.5 of gestation (E14.5). On the other hand, female mice from group D8 were fed a FAD diet for 8 wk (6 wk before conception and during the first 2 wk of pregnancy). For the data analysis, we first estimated the incidence of malformations in each group. Then, the statistical analysis was performed using IBM SPSS Statistics, version 25.0. Expression patterns of collagen IV and laminin-1 were examined with the immunohistochemical technique.

RESULTS

Our results showed that mice born to FA-deficient mothers had several congenital eye abnormalities. Embryos from dams fed a short-term FAD diet were found to have many significant abnormalities in both anterior and posterior segments, as well as choroidal vessel abnormalities. However, embryos from dams fed a long-term FAD diet had a significantly higher incidence of eye defects. Finally, maternal FA deficiency increased the expression of both collagen IV and laminin-1. Likewise, changes in the spatial localization and organization of collagen IV were observed.

CONCLUSIONS

A maternal FAD diet for a short-term period causes eye developmental defects and induces overexpression of both collagen IV and laminin-1. The malformations observed are probably related to alterations in the expression of basement membrane proteins.

Congenital aniridia - A comprehensive review of clinical features and therapeutic approaches

Congenital aniridia is a rare genetic eye disorder with total or partial absence of the iris from birth. In most cases the genetic origin of aniridia is a mutation in the PAX6 gene, leading to involvement of most eye structures. Hypoplasia of the fovea is usually present and is associated with reduced visual acuity and nystagmus. Aniridia-associated keratopathy, glaucoma, and cataract are serious and progressive complications that can further reduce visual function. Treatment of the ocular complications of aniridia is challenging and has a high risk of side effects. New approaches such as stem cell therapy may, however, offer better prognoses. We describe the various ocular manifestations of aniridia, with a special focus on conditions that commonly require treatment. We also review the growing literature reporting systemic manifestations of the disease.

Meibomian gland dysfunction and keratopathy are associated with dry eye disease in aniridia

Aims

To investigate the aetiology and characteristics of dry eye disease (DED) in a Nordic cohort of patients with congenital aniridia.

Methods

Thirty-four Norwegian and one Danish subject with congenital aniridia and 21 healthy controls were examined. All subjects underwent an extensive dry eye examination, including evaluation of meibomian glands (MGs) by meibography, measurement of tear production and tear film osmolarity and grading of vital staining of the ocular surface. Moreover, slit-lamp biomicroscopy was undertaken, including grading of aniridia-associated keratopathy (AAK).

Results

Mean tear film osmolarity was significantly higher (314±11 mOsmol/L) in patients with aniridia compared with the healthy control group (303±11 mOsmol/L, p=0.002). Vital staining score was higher in the aniridia group (4.3±3.0) compared with healthy controls (2.4±1.6, p=0.02). The degree of staining correlated positively with the stage of AAK (r=0.44, p=0.008) and negatively with corneal sensitivity (r=−0.45, p=0.012). Number of expressible MGs was lower in aniridia subjects (2.9±1.6) than in controls (4.0±1.3, p=0.007). MG loss, staged from 0 to 3, was higher in the aniridia group than in the control group, both in upper eyelid (0.86±0.89 vs 0.10±0.31, p=0.001) and lower eyelid (0.94±0.73 vs 0.30±0.47, p=0.003). Computerised analyses showed thinning (p=0.004) and lower density (p<0.001) of the MGs compared with the healthy population.

Conclusions

Patients with congenital aniridia demonstrate increased tear film osmolarity, ocular surface staining, loss of MGs and lower MG expressibility. We conclude that meibomian gland dysfunction and keratopathy are related to development of DED in aniridia.

Neuronal differentiation in the early human retinogenesis

AIM

Our study investigates the differentiation of retinal stem cells towards different neuronal subtypes during the critical period of human eye development.

METHODS

Expression of the neuronal marker neurofilament 200 (NF200), tyrosine hydroxilase (TH) and choline acetyltransferase (ChAT) was seen by immunofluorescence in the 5th-12th - week stage of development in the human eye. Data was analysed by Mann-Whitney, Kruskal-Wallis and Dunn's post hoc tests.

RESULTS

NF200, TH and ChAT cells appeared in the 5th/6th week and gradually increased during further development. The proportion of TH positive areas were distributed similarly to NF200, with a higher proportion in the outer neuroblastic layer. The proportion of a ChAT positive surface was highest in the 5th/6th - week whilst from the 7th week onwards, its proportion became higher in the optic nerve and inner neuroblastic layers than in the outer layer, where a decrease of ChAT positive areas were seen.

CONCLUSIONS

Our study indicates a high differentiation potential of early retinal cells, which decreased with the advancement of development. The observed great variety of retinal phenotypic expressions results from a large scale of influences, taking place at different developmental stages.

Sperm-borne miRNAs and endo-siRNAs are important for fertilization and preimplantation embryonic development

Although it is believed that mammalian sperm carry small noncoding RNAs (sncRNAs) into oocytes during fertilization, it remains unknown whether these sperm-borne sncRNAs truly have any function during fertilization and preimplantation embryonic development. Germline-specific Dicer and Drosha conditional knockout (cKO) mice produce gametes (i.e. sperm and oocytes) partially deficient in miRNAs and/or endo-siRNAs, thus providing a unique opportunity for testing whether normal sperm (paternal) or oocyte (maternal) miRNA and endo-siRNA contents are required for fertilization and preimplantation development. Using the outcome of intracytoplasmic sperm injection (ICSI) as a readout, we found that sperm with altered miRNA and endo-siRNA profiles could fertilize wild-type (WT) eggs, but embryos derived from these partially sncRNA-deficient sperm displayed a significant reduction in developmental potential, which could be rescued by injecting WT sperm-derived total or small RNAs into ICSI embryos. Disrupted maternal transcript turnover and failure in early zygotic gene activation appeared to associate with the aberrant miRNA profiles in Dicer and Drosha cKO spermatozoa. Overall, our data support a crucial function of paternal miRNAs and/or endo-siRNAs in the control of the transcriptomic homeostasis in fertilized eggs, zygotes and two-cell embryos. Given that supplementation of sperm RNAs enhances both the developmental potential of preimplantation embryos and the live birth rate, it might represent a novel means to improve the success rate of assisted reproductive technologies in fertility clinics.

Interplay of proliferation and differentiation factors is revealed in the early human eye development

BACKGROUND

Eye development is a consequence of numerous epithelial-to-mesenchymal interactions between the prospective lens ectoderm, outpocketings of the forebrain forming optic vesicles, and surrounding mesenchyme. How different cell types forming eye structures differentiate from their precursors, and which factors coordinate complex human eye development remains largely unknown. Proper differentiation of photoreceptors is of special interest because of their involvement in the appearance of degenerative retinal diseases.

METHODS

Here we analyze the spatiotemporal expression of neuronal markers nestin, protein gene product 9.5 (PGP9.5), and calcium binding protein (S100), proliferation marker (Ki-67), markers for cilia (alpha-tubulin), and cell stemness marker octamer-binding transcription factor 4 (Oct-4) in histological sections of 5-12 -week human eyes using immunohistochemical and immunofluorescence methods.

RESULTS

While during the investigated developmental period nestin shows strong expression in all mesenchymal derivatives, lens, optic stalk and inner neuroblastic layer, PGP9.5 and S100 expression characterizes only neural derivatives (optic nerve and neural retina). PGP9.5 is co-localized with nestin and S100 in the differentiating cells of the inner neuroblastic layer. Initially strong proliferation in all parts of the developing eye gradually ceases, especially in the outer neuroblastic layer. Proliferating Ki-67 positive cells co-localize with nestin in the retina, lens, and choroid. Strong Oct-4 and alpha-tubulin immunoreactivity in the retina and optic nerve gradually decreases, while they co-localize in outer neuroblastic and nerve fiber layers.

CONCLUSIONS

The described expression of investigated markers indicates their importance in eye growth and morphogenesis, while their spatially and temporally restricted pattern coincides with differentiation of initially immature cells into specific retinal cell lineages. Alterations in their spatiotemporal interplay might lead to disturbances of visual function.

Aberrant activation of p53 due to loss of MDM2 or MDMX causes early lens dysmorphogenesis

Although forming a heterodimer or heterooligomer is essential for MDM2 and MDMX to fully control p53 during early embryogenesis, deletion of either MDM2 or MDMX in specific tissues using the loxp-Cre system reveals phenotypic diversity during organ morphogenesis, which can be completely rescued by loss of p53, suggesting the spatiotemporal independence and specificity of the regulation of p53 by MDM2 and MDMX. In this study, we investigated the role of the MDM2-MDMX-p53 pathway in the developing lens that is a relatively independent region integrating cell proliferation, differentiation and apoptosis. Using the mice expressing Cre recombinase specifically in the lens epithelial cells (LECs) beginning at E9.5, we demonstrated that deletion of either MDM2 or MDMX induces apoptosis of LEC and reduces cell proliferation, resulting in lens developmental defect that finally progresses into aphakia. Specifically, the lens defect caused by MDM2 deletion was evident at E10, occurring earlier than that caused by MDMX deletion. These lens defects were completely rescued by loss of two alleles of p53, but not one allele of p53. These results demonstrate that both MDM2 and MDMX are required for monitoring p53 activity during lens development, and they may function independently or synergistically to control p53 and maintain normal lens morphogenesis.

Congenital abnormalities of the optic nerve: from gene mutation to clinical expression

Optic nerve malformations are common causes of congenital blindness and are recognized with increasing prevalence. The importance of identifying these malformations lies not only in determining the cause and level of visual impairment, but also in looking for associated treatable or life threatening systemic conditions. A number of genetic mutations have been identified in the development of optic disc anomalies, such as ones in PAX2 or PAX6. Recent advances in genetic testing like array comparative genomic hybridization allow the detection of microdeletions that were previously difficult or impossible to identify.

Excess caffeine exposure impairs eye development during chick embryogenesis

Caffeine has been an integral component of our diet and medicines for centuries. It is now known that over consumption of caffeine has detrimental effects on our health, and also disrupts normal foetal development in pregnant mothers. In this study, we investigated the potential teratogenic effect of caffeine over-exposure on eye development in the early chick embryo. Firstly, we demonstrated that caffeine exposure caused chick embryos to develop asymmetrical microphthalmia and induced the orbital bone to develop abnormally. Secondly, caffeine exposure perturbed Pax6 expression in the retina of the developing eye. In addition, it perturbed the migration of HNK-1⁺ cranial neural crest cells. Pax6 is an important gene that regulates eye development, so altering the expression of this gene might be the cause for the abnormal eye development. Thirdly, we found that reactive oxygen species (ROS) production was significantly increased in eye tissues following caffeine treatment, and that the addition of anti-oxidant vitamin C could rescue the eyes from developing abnormally in the presence of caffeine. This suggests that excess ROS induced by caffeine is one of the mechanisms involved in the teratogenic alterations observed in the eye during embryogenesis. In sum, our experiments in the chick embryo demonstrated that caffeine is a potential teratogen. It causes asymmetrical microphthalmia to develop by increasing ROS production and perturbs Pax6 expression.

A comparative analysis of Müller glia-mediated regeneration in the vertebrate retina

This article reviews the current state of knowledge regarding the potential of Müller glia to become neuronal progenitor cells in the avian retina. We compare and contrast the remarkable proliferative and neurogenic capacity of Müller glia in the fish retina to the limited capacity of Müller glia in avian and rodent retinas. We summarize recent findings regarding the secreted factors, signaling pathways and cell intrinsic factors that have been implicated in the formation of Müller glia-derived progenitors. We discuss several key similarities and differences between the fish, rodent and chick model systems, highlighting several of the key transcription factors and signaling pathways that regulate the formation of Müller glia-derived progenitors.

Lens and retina regeneration: new perspectives from model organisms

Comparative studies of lens and retina regeneration have been conducted within a wide variety of animals over the last 100 years. Although amphibians, fish, birds and mammals have all been noted to possess lens- or retina-regenerative properties at specific developmental stages, lens or retina regeneration in adult animals is limited to lower vertebrates. The present review covers the newest perspectives on lens and retina regeneration from these different model organisms with a focus on future trends in regeneration research.

Retinal dopamine mediates multiple dimensions of light-adapted vision

Dopamine is a key neuromodulator in the retina and brain that supports motor, cognitive, and visual function. Here, we developed a mouse model on a C57 background in which expression of the rate-limiting enzyme for dopamine synthesis, tyrosine hydroxylase, is specifically disrupted in the retina. This model enabled assessment of the overall role of retinal dopamine in vision using electrophysiological (electroretinogram), psychophysical (optokinetic tracking), and pharmacological techniques. Significant disruptions were observed in high-resolution, light-adapted vision caused by specific deficits in light responses, contrast sensitivity, acuity, and circadian rhythms in this retinal dopamine-depleted mouse model. These global effects of retinal dopamine on vision are driven by the differential actions of dopamine D1 and D4 receptors on specific retinal functions and appear to be due to the ongoing bioavailability of dopamine rather than developmental effects. Together, our data indicate that dopamine is necessary for the circadian nature of light-adapted vision as well as optimal contrast detection and acuity.

Zebrafish cadherin-11 participates in retinal differentiation and retinotectal axon projection during visual system development

BACKGROUND

Cadherins orchestrate tissue morphogenesis by controlling cell adhesion, migration and differentiation. Various cadherin family members are expressed in the retina and other neural tissues during embryogenesis, regulating development of these tissues. Cadherin-11 (Cdh11) is expressed in mesenchymal, bone, epithelial, neural and other tissues, and this cadherin was shown to control cell migration and differentiation in neural crest, tumor and bone cells. Our previous studies characterized Cdh11 expression and function in zebrafish.

RESULTS

Here, we report effects of Cdh11 loss-of-function on visual system development using morpholino oligonucleotide knockdown methods. Cdh11 is expressed in the retina and lens during retinal differentiation. Cdh11 loss-of-function produced defects in retinal differentiation and lens development. Cdh11 loss-of-function also reduced retinotectal axon projection and organization, consistent with known Cdh11 function in cell migration.

CONCLUSION

Cdh11 expression in the developing visual system and Cdh11 loss-of-function phenotype illustrates the critical role for differential cadherin activity in visual system differentiation and organization.

Roles for Dicer1 in the patterning and differentiation of the optic cup neuroepithelium

The embryonic ocular neuroepithilium generates a myriad of cell types, including the neuroretina, the pigmented epithelium, the ciliary and iris epithelia, and the iris smooth muscles. As in other regions of the developing nervous system, the generation of these various cell types requires a coordinated sequence of patterning, specification and differentiation events. We investigated the roles of microRNAs (miRNAs) in the development of optic cup (OC)-derived structures. We inactivated Dicer1, a key mediator of miRNA biosynthesis, within the OC in overlapping yet distinct spatiotemporal patterns. Ablation of Dicer1 in the inner layer of the OC resulted in patterning alteration, particularly at the most distal margins. Following loss of Dicer1, this region generated a cryptic population of cells with a mixed phenotype of neuronal and ciliary body (CB) progenitors. Notably, inactivation of Dicer1 in the retinal progenitors further resulted in abrogated neurogenesis, with prolongation of ganglion cell birth and arrested differentiation of other neuronal subtypes, including amacrine and photoreceptor cells. These alterations were accompanied by changes in the expression of Notch and Hedgehog signaling components, indicating the sensitivity of the pathways to miRNA activity. Moreover, this study revealed the requirement of miRNAs for morphogenesis of the iris and for the regulation of CB cell type proliferation and differentiation. Together, analysis of the three genetic models revealed novel, stage-dependent roles for miRNAs in the development of the ocular sub-organs, which are all essential for normal vision.

Stem cells of the human cornea

INTRODUCTION

Blindness affects 50 million individuals worldwide; a significant proportion of them require a cell or tissue-based repair or replacement strategy to mend their damaged or diseased cornea. This review will focus on the epithelial stem cell (ESC) population of the cornea, where they reside, how they are identified and what alternative cells can be used as functional substitutes.

SOURCE OF DATA

Data for this review were collated after performing literature searches by inserting key words (cornea, limbal, stem cells (SCs), epithelium, stroma, and endothelium) into the search engine PubMed.

AREAS OF AGREEMENT

The prevailing notion is that corneal ESCs reside in an exclusive niche and their activation is dictated by niche-specific signals.

AREAS OF CONTROVERSY

Recent studies refute this hypothesis, as the central cornea of many animal species also possesses similar proliferative and clonogenic activity. The other area of controversy is in relation to the use of autologous and/or allogeneic cell therapies which are mostly contaminated with xenogeneic factors, potentially exposing the recipient to potentially harmful foreign infection.

GROWING POINTS

Due to the shortage of donor corneal biomaterial, alternative cellular sources are being sought, discovered and trialed.

AREAS TIMELY FOR DEVELOPING RESEARCH

With the exception of conjunctival and oral mucosal epithelium, no other cell type has been successfully used to treat patients with severe corneal epithelial defects. Embryonic and foetal SCs may have the greatest potential of all; however there are moral, legal, religious and scientific hurdles to overcome before they become routinely used in the clinic.

Time-specific blockade of PDGFR with Imatinib (Glivec®) causes cataract and disruption of lens fiber cells in neonatal mice

This study aimed at investigating the response of lens epithelial cells in postnatal mice to Imatinib (Glivec®, a potent inhibitor of platelet-derived growth factor receptor (PDGFR)) treatment. Mouse eyes were sampled 10 days after administration of Imatinib (0.5 mg·g(-1)·day(-1)) for 3 days, at either 7, 14, or 21 days postpartum. Structural changes of lens were revealed by routine H.E. staining. Levels of proliferation and apoptosis were revealed by BrdU incorporation and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, respectively, and immunofluorescent staining with anti-PDGFRα antibody was carried out on the sections of eyeball. PDGFRα and p-PDGFRαprotein levels were evaluated by Western blot. Our results indicated that administration of Imatinib led to blockade of PDGFR signaling. Formation of cataracts was found only in those mice where treatment started from 7 days postpartum (P7), but was not observed in those samples from P14 nor P21. Fiber cells were disorganized in cataract lens core as observed histologically, and migration of epithelial cells was also inhibited. No apoptosis was detected with the TUNEL method. Our results indicated blockade of PDGFR at the neonatal stage (P7) would lead to cataracts and lens fiber cells disorganization, suggesting that PDGFR signaling plays a time-specific and crucial role in the postnatal development of lens in the mouse, and also may provide a new approach to produce a congenital cataract animal model.

Molecular and cellular aspects of amphibian lens regeneration

Lens regeneration among vertebrates is basically restricted to some amphibians. The most notable cases are the ones that occur in premetamorphic frogs and in adult newts. Frogs and newts regenerate their lens in very different ways. In frogs the lens is regenerated by transdifferentiation of the cornea and is limited only to a time before metamorphosis. On the other hand, regeneration in newts is mediated by transdifferentiation of the pigment epithelial cells of the dorsal iris and is possible in adult animals as well. Thus, the study of both systems could provide important information about the process. Molecular tools have been developed in frogs and recently also in newts. Thus, the process has been studied at the molecular and cellular levels. A synthesis describing both systems was long due. In this review we describe the process in both Xenopus and the newt. The known molecular mechanisms are described and compared.

Corneal stem cells and their origins: significance in developmental biology

Adult corneal stem cells (SCs) have been the subject of substantial research over the past 2 decades, with promising clinical applications being devised, refined, and tried. However, there have been few studies on the early development of these cells in humans, perhaps due to ethical and practical constraints. This review highlights work that has yielded significant insights from developmental studies in the cornea and other SC repositories. This field merits further research to improve our current knowledge of the origin of SCs, their location, phenotype, function, and niche structure, as well as providing fresh insight into the pathogenesis of congenital diseases and new therapeutic avenues for treating a range of blinding corneal diseases.

Seven kinds of intermediate filament networks in the cytoplasm of polarized cells: structure and function

Intermediate filaments (IFs) are involved in many important physiological functions, such as the distribution of organelles, signal transduction, cell polarity and gene regulation. However, little information exists on the structure of the IF networks performing these functions. We have clarified the existence of seven kinds of IF networks in the cytoplasm of diverse polarized cells: an apex network just under the terminal web, a peripheral network lying just beneath the cell membrane, a granule-associated network surrounding a mass of secretory granules, a Golgi-associated network surrounding the Golgi apparatus, a radial network locating from the perinuclear region to the specific area of the cell membrane, a juxtanuclear network surrounding the nucleus, and an entire cytoplasmic network. In this review, we describe these seven kinds of IF networks and discuss their biological roles.

Vertebrates that regenerate as models for guiding stem cels

There are several animal model organisms that have the ability to regenerate severe injuries by stimulating local cells to restore damaged and lost organs and appendages. In this chapter, we will describe how various vertebrate animals regenerate different structures (central nervous system, heart and appendages) as well as detail specific cellular and molecular features concerning the regeneration of these structures.

Ocular abnormalities in mice lacking the immunoglobulin superfamily member Cdo

Vertebrate eye development requires a series of complex morphogenetic and inductive events to produce a lens vesicle centered within the bilayered optic cup and a posteriorly positioned optic stalk. Multiple congenital eye defects, including microphthalmia and coloboma, result from defects in early eye morphogenesis. Cdo is a multifunctional cell surface immunoglobulin superfamily member that interacts with and mediates signaling by cadherins and netrins to regulate myogenesis. In addition, Cdo plays an essential role in early forebrain development by functioning as coreceptor for sonic hedgehog. It is reported here that Cdo is expressed in a dynamic, but dorsally restricted, fashion during early eye development, and that mice lacking Cdo display multiple eye defects. Anomalies seen in Cdo(-/-) mice include coloboma (failure to close the optic fissure); failure to form a proper boundary between the retinal pigmented epithelium and optic stalk; defective lens formation, including failure to separate from the surface ectoderm; and microphthalmia. Consistent with this wide array of defects, developing eyes of Cdo(-/-) mice show altered expression of several regulators of dorsoventral eye patterning, including Pax6, Pax2, and Tbx5. Taken together, these findings show that Cdo is required for normal eye development and is required for normal expression of patterning genes in both the ventral and dorsal domains. The multiple eye development defects seen in Cdo(-/-) mice suggest that mutations in human Cdo could contribute to congenital eye anomalies, such as Jacobsen syndrome, which is frequently associated with ocular defects, including coloboma and Peters' anomaly.

Gene expression profiles of lens regeneration and development in Xenopus laevis

Seven hundred and thirty-four unique genes were recovered from a cDNA library enriched for genes up-regulated during the process of lens regeneration in the frog Xenopus laevis. The sequences represent transcription factors, proteins involved in RNA synthesis/processing, components of prominent cell signaling pathways, genes involved in protein processing, transport, and degradation (e.g., the ubiquitin/proteasome pathway), matrix metalloproteases (MMPs), as well as many other proteins. The findings implicate specific signal transduction pathways in the process of lens regeneration, including the FGF, TGF-beta, MAPK, Retinoic acid, Wnt, and hedgehog signaling pathways, which are known to play important roles in eye/lens development and regeneration in various systems. In situ hybridization revealed that the majority of genes recovered are expressed during embryogenesis, including in eye tissues. Several novel genes specifically expressed in lenses were identified. The suite of genes was compared to those up-regulated in other regenerating tissues/organisms, and a small degree of overlap was detected.

Current Prospects for Adult Stem Cell–Based Therapies in Ocular Repair and Regeneration

Recent advances in stem cell biology have led to the exploration of stem cell-based therapies to treat a wide range of human diseases. In the ophthalmic field, much hope has been placed on the potential use of these cells to restore sight, particularly in those conditions in which other established treatments have failed and in which visual function has been irreversibly damaged by disease or injury. At present, there are many limitations for the immediate use of embryonic stem cells to treat ocular disease, and as more evidence emerges that adult stem cells are present in the adult human eye, it is clear that these cells may have advantages to develop into feasible therapeutic treatments without the problems associated with embryonic research and immune rejection. Here we discuss the current prospects for the application of various adult ocular stem cells to human therapies for restoration of vision.

Childhood visual impairment: normal and abnormal visual function in the context of developmental disability

Abnormal or failed development of vision in children may give rise to varying degrees of visual impairment and disability. Disease and organ-specific mechanisms by which visual impairments arise are presented. The presentation of these mechanisms, along with an explanation of established pathologic processes and correlative up-to-date clinical and social research in the field of pediatrics, ophthalmology, and rehabilitation medicine are discussed. The goal of this article is to enhance the practitioner's recognition and care for children with developmental disability associated with visual impairment.

Identification of the minimal promoter region of the mouse NKX5-3, a transcription factor implicated in eye development

Early ocular development is controlled by a complex network of transcription factors, cell cycle regulators, and diffusible signalling molecules. Together, these molecules regulate cell proliferation and apoptosis, and specify retinal fate. NKX5-3 is a homeobox transcription factor implicated in eye development. The analysis of the 5'-flanking region of the mouse Nkx5-3 gene revealed a predicted TATA-less promoter sequence between -416 and -166 of the translation start site. To functionally characterise Nkx5-3 promoter activity, serial deletions of the promoter sequence were introduced in pGL-3 basic vector and promoter activity of these 5'- and 3'-deleted constructions was tested in HeLa and CHO cells. Transactivation assays identified a region between -350 and -296 exhibiting promoter-like activity. Combined analysis by deletions and point mutations showed that this sequence, containing multiple Sp1 binding sites was necessary to promote transcriptional activity. Binding of Sp1 to this region was confirmed by electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation, using an antibody specific for Sp1. Altogether, these results demonstrated that the immediate upstream region of Nkx5-3 gene possessed a strong intrinsic promoter activity in vitro, suggesting a potential role in Nkx5-3 transcription in vivo.

Molecular events in early development of the ciliary body: a question of folding

Ciliary body morphogenesis is a complicated, multi-step process requiring coordinated changes in cell shape, flexure of epithelial sheets and dynamic shifts in mitotic rates. Very little is known of how these cellular events are triggered or regulated. This review summarises current models of ciliary body morphogenesis. The role of intraocular pressure as a driver of morphogenesis is re-evaluated in the light of new information. An update on the role of the lens in ciliary body morphogenesis is presented. In the second part of the review current gene expression data is related to ciliary body morphogenesis. In particular the role of Bmp4 and its downstream target genes are discussed, with novel gene expression patterns of Bmp4 and Tgfbeta1i4 being presented.

Mapping of genetic modifiers affecting the eye phenotype of ocular retardation (Chx10or-J) mice

Ocular retardation is a recessive murine mutation whose phenotypic expression is greatly affected by genetic background effects. Mice of the inbred 129/SvJ background that are homozygous for the Chx10(or-J) mutation are blind and have a thin, poorly differentiated retina and no optic nerve. A backcross between 129/SvJ and Mus musculus castaneus (CASA/Rk) produced animals that were homozygous for the Chx10(or-J) mutation, yet showed a much milder phenotype. Such animals, when brother-sister mated and selected for mild phenotype for several generations, resulted in partial recovery of visual function, including presence of an optic nerve and pupillary response. In this article we report a genome scan of phenotypic extremes of the backcross to identify the genetic loci affecting this phenotype modification. Our scan revealed significant loci on Chromosomes 6 and 14 where the CASA/Rk alleles are maintained selectively. Markers were developed near candidate genes, but no candidate gene could be identified unequivocally.

Postnatal development of the eye in the naked mole rat (Heterocephalus glaber)

The naked mole rat (Heterocephalus glaber) is a subterranean rodent whose eyes are thought to be visually nonfunctional and as such is an ideal animal with which to pursue questions in evolutionary developmental biology. This report is the first in-depth study on the development and morphology of the naked mole rat eye. Using standard histological analysis and scanning and transmission electron microscopy, we describe the structural features of the eye. We further report on the morphological changes that accompany the development of this eye from neonate to adult and compare them with those that occur during mouse eye development. We observed numerous abnormalities in the shape and cellular arrangement of the structures of the anterior chamber, with notable malformations of the lens. Cell proliferation and cell death assays were conducted to investigate the possible causes of lens malformation. We found that neither of these processes appeared abnormal, indicating that they were not responsible for the lens phenotype of the mole rat. In order to investigate the process of lens differentiation, we analyzed the expression of gamma-crystallins using Western blots and immunocytochemistry. At birth, levels of gamma-crystallin appear normal, but soon thereafter, the gamma-crystallin expression is terminated. Absence of detectable gamma-crystallins in adults suggests that there is a gradual degradation and loss of these proteins. The evolutionary factors that could be responsible for the eye morphology of the naked mole rat are discussed. A model for abnormal lens differentiation and the role it plays in the morphogenesis of the rest of the eye in the naked mole rats is proposed.

Neural regeneration in the chick retina

In warm-blooded vertebrates, possibilities for retinal regeneration have recently become reality with the discovery of neural stem cells in the mature eye. A number of different cellular sources of neural stem cells have been identified. These sources include stem cells at the retinal margin, pigmented cells in the ciliary body and iris, non-pigmented cells in the ciliary body and Müller glia within the retina. This review focuses on recent reports of neural stem cells and regeneration in the postnatal chicken retina. In the chicken eye sources of neurogensis and regeneration include: (1) retinal stem cells at the peripheral edge of the retina; (2) Müller glia in central regions of the retina; (3) non-pigmented epithelial cells in the posterior portion of the ciliary body; and (4) possibly pigmented cells in the pars plana of the ciliary body. This review discusses the similarities between the retinal progenitor cells in the postnatal eye and those found in the embryo. In addition, I discuss combinations of growth factors, (insulin, IGF-I, EGF and FGF2) that are capable of stimulating the proliferation and production of neurons from neural progenitors, non-neural epithelial cells, and postmitotic support cells in the avian eye. In summary, the mechanisms that regulate the proliferation and differentiation of cells with neurogenic potential are beginning to be understood and the postnatal chicken eye has proven to be a useful model system to study retinal regeneration.

Aniridia and optic nerve hypoplasia

BACKGROUND

This study evaluates, in patients with aniridia, the prevalence of optic nerve hypoplasia and its association with foveal hypoplasia.

METHODS

The medical records of 56 patients with aniridia (31 female, 25 male, mean age 33 years, range 2-74 years) were retrospectively evaluated for optic nerve and foveal hypoplasia. The difference in prevalence of foveal hypoplasia in patients with and without optic nerve hypoplasia was compared using Fisher's exact test.

RESULTS

Six of 56 patients, 10.7% (95% CI: 4.8-21.5%), had optic nerve hypoplasia; hypoplasia was found in both eyes of five binocular patients and in one monocular patient. The prevalence of foveal hypoplasia was higher in aniridia patients with optic nerve hypoplasia than in those without (50.0 vs 6.0%); this difference did not achieve statistical significance (P=0.10).

CONCLUSIONS

Clinically apparent optic nerve hypoplasia is found in roughly 10% of patients with aniridia and may occur independently or in association with foveal hypoplasia.

Spondylo-ocular syndrome: a new entity with crystalline lens malformation, cataract, retinal detachment, osteoporosis, and platyspondyly

PURPOSE

To define a new clinical entity in a consanguineous family with six children affected by a spondylo-ocular syndrome, including cataract, crystalline lens malformation, retinal detachment, osteoporosis, and platyspondyly. To analyze candidate genes of connective tissue disorders as a possible underlying disorder and to demonstrate especially the ocular phenotype.

DESIGN

Observational case series.

METHODS

Consanguineous parents, one unaffected sibling and five affected children with clinical features of spondylo-ocular syndrome, were demonstrated. Clinical examination, radiologic, laboratory, and cytogenetic as well as moleculargenetic analyses were performed. The segregation of flanking marker alleles of three collagen genes and the interval for osteoporosis-pseudoglioma syndrome were analyzed. Two microsatellite markers located within Pax6CA/GT region were tested for homozygosity.

RESULTS

On laboratory investigation a normal excretion of amino acids, mucopolysaccharides, and oligosaccharides could be found. The karyotype was normal. Complete radiologic examination in one index patient revealed a generalized moderate osteoporosis, platyspondyly with fish bone appearance, and greatly enlarged intervertebral spaces. The candidate genes known to be in Stickler syndrome as well as linkage to the osteoporosis-pseudoglioma syndrome candidate region could be excluded. None of the affected showed homozygosity for the Pax6 microsatellite markers.

CONCLUSIONS

We conclude that the phenotype and the clinical features in this family defines a new Mendelian disorder. It remains to be seen what kind of molecule shared by eye and bone is involved.

Developmental genetics in ophthalmology

Much of our knowledge about the function of genes in mammalian development has been derived from the molecular analysis of spontaneous or induced mutations in the mouse. Since mutations affecting the mouse eye can be easily identified, a remarkable number of mutant lines provide animal models for congenital anomalies in man. To understand the mechanisms of lens development in detail, the isolation of the corresponding genes and the characterization of the mutations at the molecular level are important. A prerequisite for molecular analysis is the chromosomal localization of the gene. In this review, some mutants from our institute will be discussed according to the embryological time scale of the expression of the affected genes, reflecting also their genetic hierarchy. (1) In the aphakia mouse mutant, two deletions in the promoter of the homeobox transcription factor Pitx3 lead to a loss of its function and to an arrest of eye development at the lens stalk stage. Mutations in the homologous human PITX3 gene have been demonstrated to be causative of cataracts and the dysmorphology of the anterior segment of the eye. (2) Connexin50 is present in the lens vesicle. Later on, it becomes abundant in the anterior part of the fiber cells and in the lens epithelial cells. Mutations in the connexin50-encoding gene Gja8 lead to dominant cataracts. (3) alphaA-crystallin is present in the mouse lens cup, in the posterior half of the lens vesicle, and later in a high concentration in the lens fiber cells. Mutations in the alphaA-crystallin-encoding gene Cryaa lead to recessive and dominant cataracts. (4) Mutations in the gamma-crystallin -encoding genes (Cryg) are the most frequent cause of congenital, dominant nuclear, or total cataracts in the mouse. Indications from our first studies in congenital human cataracts support these data. (5) Some postnatal, progressive cataracts have been characterized by mutations in the beta-crystallin -encoding genes (Cryb). Since at least one of them is also expressed in the retina and the brain, effects on these tissues have to be considered, too.

The stability of the lens-specific Maf protein is regulated by fibroblast growth factor (FGF)/ERK signaling in lens fiber differentiation

Fibroblast growth factor (FGF) signaling is necessary for both proliferation and differentiation of lens cells. However, the molecular mechanisms by which FGFs exert their effects on the lens remain poorly understood. In this study, we show that FGF-2 repressed the expression of lens-specific genes at the proliferative phase in primary cultured lens cells. Using transfected cells, we also found that the activity of L-Maf, a lens differentiation factor, is repressed by FGF/ERK signaling. L-Maf is shown to be phosphorylated by ERK, and introduction of mutations into the ERK target sites on L-Maf promotes its stabilization. The stable L-Maf mutant protein promotes the differentiation of lens cells from neural retina cells. Taken together, these results indicate that FGF/ERK signaling negatively regulates the function of L-Maf in proliferative lens cells and that stabilization of the L-Maf protein is important for lens fiber differentiation.

The homeobox gene Six3 is a potential regulator of anterior segment formation in the chick eye

The anterior segment of the vertebrate eye consists of highly organized and specialized ocular tissues critical for normal vision. The periocular mesenchyme, originating from the neural crest, contributes extensively to the anterior segment. During chick eye morphogenesis, the homeobox gene Six3 is expressed in a subset of periocular mesenchymal cells and in differentiating anterior segment tissues. Retrovirus-mediated misexpression of Six3 causes eye anterior segment malformation, including corneal protrusion and opacification, ciliary body and iris hypoplasia, and trabecular meshwork dysgenesis. Histological and molecular marker analyses demonstrate that Six3 misexpression disrupts the integrity of the corneal endothelium and the expression of extracellular matrix components critical for corneal transparency. Six3 misexpression also leads to a reduction of the periocular mesenchymal cell population expressing Lmx1b, Pitx2, and Pax6, transcription factors critical for eye anterior segment morphogenesis. Moreover, elevated levels of Six3 attenuate proliferation of periocular mesenchymal cells in vitro and differentiating anterior segment tissues in vivo. These results suggest that, in addition to its function in eye primordium determination, Six3 plays a role in regulating the development of the vertebrate eye anterior segment.

Molecular cloning, characterization and mapping of the canine glucocorticoid receptor DNA binding factor 1 (GRLF1)

Progressive rod-cone degeneration (prcd) is an autosomal recessive retinal degeneration of dogs that maps to chromosome 9 (CFA9). Positional cloning and candidate gene approaches are presently used to identify the disease-causing gene. To complement these strategies and identify novel candidate genes, we have used a subtraction approach to detect modified gene expression caused by prcd that may be causally associated with the disease, or, alternatively, be involved in the molecular mechanisms leading to the disease phenotype. With this technique we characterized a 4503 nucleotide open reading frame (ORF) within a 5.6 kb cDNA that predicts a protein of 1500 amino acids. The gene shows about 90% homology to the human and rat glucocorticoid receptor DNA binding factor 1 (GRLF1) gene, also known as p190-A. The transcript was detected in several tissues, including retina, and the protein was localized to the photoreceptor cell layer. The canine GRLF1 maps near the telomere of CFA1 close to CRX, a region synteny to human chromosome 19q13 (HSA19q13). Based on its chromosomal location, GRLF1 has been excluded as a candidate gene for prcd. Northern blot analysis also failed to prove down-regulation of the gene in early stages of disease in six different non-allelic canine retinal degenerations. However, we were able to show that in advanced stages of prcd, GRLF1 is expressed in remaining photoreceptor cells, thus, providing a challenging task to uncover the gene's exact function in the retina and degenerative processes.

Characterizing gene expression during lens formation in Xenopus laevis: evaluating the model for embryonic lens induction

Few directed searches have been undertaken to identify the genes involved in vertebrate lens formation. In the frog Xenopus, the larval cornea can undergo a process of transdifferentiation to form a new lens once the original lens is removed. Based on preliminary evidence, we have shown that this process shares many elements of a common molecular/genetic pathway to that involved in embryonic lens development. A subtracted cDNA library, enriched for genes expressed during cornea-lens transdifferentiation, was prepared. The similarities/identities of specific clones isolated from the subtracted cDNA library define an expression profile of cells undergoing cornea-lens transdifferentiation ("lens regeneration") and corneal wound healing (the latter representing a consequence of the surgery required to trigger transdifferentiation). Screens were undertaken to search for genes expressed during both transdifferentiation and embryonic lens development. Significantly, new genes were recovered that are also expressed during embryonic lens development. The expression of these genes, as well as others known to be expressed during embryonic development in Xenopus, can be correlated with different periods of embryonic lens induction and development, in an attempt to define these events in a molecular context. This information is considered in light of our current working model of embryonic lens induction, in which specific tissue properties and phases of induction have been previously defined in an experimental context. Expression data reveal the existence of further levels of complexity in this process and suggests that individual phases of lens induction and specific tissue properties are not strictly characterized or defined by expression of individual genes.

Retinal ablation and altered lens differentiation induced by ocular overexpression of BMP7

The alphaA-crystallin promoter was used to target expression of bone morphogenetic protein 7 (BMP7) to lens fiber cells in transgenic mice. Surprisingly, lens-specific expression of BMP7 induced widespread apoptosis and rapid ablation of the neural retina in multiple families. Subsequent to retinal ablation, the lens bow region shifted posteriorly until lens epithelial cells completely enveloped the lens. Lens-specific expression of FGF3 was found to rescue the loss of fiber cell differentiation. Our results show that elevated BMP7 levels can induce rapid retinal degeneration accompanied by disruption of the endogenous ocular system for fiber cell induction.

Early eye development in vertebrates

This review provides a synthesis that combines data from classical experimentation and recent advances in our understanding of early eye development. Emphasis is placed on the events that underlie and direct neural retina formation and lens induction. Understanding these events represents a longstanding problem in developmental biology. Early interest can be attributed to the curiosity generated by the relatively frequent occurrence of disorders such as cyclopia and anophthalmia, in which dramatic changes in eye development are readily observed. However, it was the advent of experimental embryology at the turn of the century that transformed curiosity into active investigation. Pioneered by investigators such as Spemann and Adelmann, these embryological manipulations have left a profound legacy. Questions about early eye development first addressed using tissue manipulations remain topical as we try to understand the molecular basis of this process.

Transdifferentiation of the ventral retinal pigmented epithelium to neural retina in the growth arrest specific gene 1 mutant

During eye development, retinal pigmented epithelium (RPE) and neural retina (NR) arise from a common origin, the optic vesicle. One of the early distinctions of RPE from NR is the reduced mitotic activity of the RPE. Growth arrest specific gene 1 (Gas1) has been documented to inhibit cell cycle progression in vitro (G. Del Sal et al., 1992, Cell 70, 595--607). We show here that the expression pattern of Gas1 in the eye supports its negative role in RPE proliferation. To test this hypothesis, we generated a mouse carrying a targeted mutation in the Gas1 locus. Gas1 mutant mice have microphthalmia. Histological examination revealed that the remnant mutant eyes are ingressed from the surface with minimal RPE and lens, and disorganized eyelid, cornea, and NR. Analysis of the Gas1 mutant indicates that there is overproliferation of the outer layer of optic cup (E10.5) immediately after the initial specification of the RPE. This defect is specific to the ventral region of the RPE. Using molecular markers for RPE (Mi and Tyrp2) and NR (Math5), we demonstrate that there is a gradual loss of Mi and Tyrp2 expression and an appearance of Math5 expression in the mutant ventral RPE region, indicating that this domain becomes respecified to NR. This "ectopic" NR develops as a mirror image of the normal NR and is entirely of ventral identity. Our data not only support Gas1's function in regulating cell proliferation, but also uncover an unexpected regional-specific cell fate change associated with dysregulated growth. Furthermore, we provide evidence that the dorsal and ventral RPEs are maintained by distinct genetic components.