Efficient generation of lens progenitor cells and lentoid bodies from human embryonic stem cells in chemically defined conditions

Differentiation of Induced Pluripotent Stem Cells to Lentoid Bodies Expressing a Lens Cell-Specific Fluorescent Reporter

Curative approaches for eye cataracts and other eye abnormalities, such as myopia and hyperopia currently suffer from a lack of appropriate models. Here, we present a new approach for in vitro growth of lentoid bodies from induced pluripotent stem (iPS) cells as a tool for ophthalmological research. We generated a transgenic mouse line with lens-specific expression of a fluorescent reporter driven by the alphaA crystallin promoter. Fetal fibroblasts were isolated from transgenic fetuses, reprogrammed to iPS cells, and differentiated to lentoid bodies exploiting the specific fluorescence of the lens cell-specific reporter. The employment of cell type-specific reporters for establishing and optimizing differentiation in vitro seems to be an efficient and generally applicable approach for developing differentiation protocols for desired cell populations.

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

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

Molecular characterization of mouse lens epithelial cell lines and their suitability to study RNA granules and cataract associated genes

The discovery of cytosolic RNA granule (RG) component proteins associated with human cataract has initiated investigations on post-transcriptional mechanisms of gene expression control in the lens. Application of established mouse lens epithelial cell lines (LECs) can provide rapid insights on RG function in lens cells, especially because mouse mutants in several RG components are not available. However, although these LECs represent potential reagents for such analyses, they are uncharacterized for lens gene expression or RG formation. Therefore, a detailed molecular and cellular characterization of three permanent mouse LECs 17EM15, 21EM15 and αTN4 is performed in this study. Comparative analysis between microarray gene expression datasets on LEC 21EM15 and iSyTE lens tissue demonstrates that 30% of top 200 iSyTE identified lens-enriched genes are expressed in these cells. Majority of these candidates are independently validated to either have lens expression, function or linkage to cataract. Moreover, analysis of microarray data with genes described in Cat-Map, an online database of cataract associated genes and loci, demonstrates that 131 genes linked to cataract loci are expressed in 21EM15 cells. Furthermore, gene expression in LECs is compared to isolated lens epithelium or fiber cells by qRT-PCR and by comparative analyses with publically available epithelium or fiber-specific microarray and RNA-seq (sequencing) datasets. Expression of select candidate genes was validated by regular and real-time quantitative RT-PCR. Expression of lens epithelium-enriched genes Foxe3, Pax6, Anxa4 and Mcm4 is up-regulated in LEC lines, compared to isolated lens fiber cells. Moreover, similar to isolated lens epithelium, all three LECs exhibit down-regulation of fiber cell-expressed genes Crybb1, Mip and Prox1 when compared to fiber cells. These data indicate that the LEC lines exhibit greater similarity to lens epithelium than to fiber cells. Compared to non-lens cell line NIH3T3, LECs exhibit significantly enriched expression of transcription factors with important function in the lens, namely Pax6, Foxe3 and Prox1. In addition to these genes, all three LECs also express key lens- and cataract-associated genes, namely Dkk3, Epha2, Hsf4, Jag1, Mab21l1, Meis1, Pknox1, Pou2f1, Sfrp1, Sparc, Tdrd7 and Trpm3. Additionally, 21EM15 microarrays indicate expression of Chmp4b, Cryab and Tcfap2a among others important genes. Immunostaining with makers for Processing bodies (P-bodies) and Stress granules (SGs) demonstrates that these classes of RGs are robustly expressed in all three LECs. Moreover, under conditions of stress, 17EM15 and αTN4 exhibit significantly higher numbers of P-bodies and SGs compared to NIH3T3 cells. In sum, these data indicate that mouse LECs 21EM15, 17EM15 and αTN4 express key lens or cataract genes, are similar to lens epithelium than fiber cells, and exhibit high levels of P-bodies and SGs, indicating their suitability for investigating gene expression control and RG function in lens-derived cells.

Pax6- and Six3-mediated induction of lens cell fate in mouse and human ES cells

Embryonic stem (ES) cells provide a potentially useful in vitro model for the study of in vivo tissue differentiation. We used mouse and human ES cells to investigate whether the lens regulatory genes Pax6 and Six3 could induce lens cell fate in vitro. To help assess the onset of lens differentiation, we derived a new mES cell line (Pax6-GFP mES) that expresses a GFP reporter under the control of the Pax6 P0 promoter and lens ectoderm enhancer. Pax6 or Six3 expression vectors were introduced into mES or hES cells by transfection or lentiviral infection and the differentiating ES cells analyzed for lens marker expression. Transfection of mES cells with Pax6 or Six3 but not with other genes induced the expression of lens cell markers and up-regulated GFP reporter expression in Pax6-GFP mES cells by 3 days post-transfection. By 7 days post-transfection, mES cell cultures exhibited a>10-fold increase over controls in the number of colonies expressing γA-crystallin, a lens fiber cell differentiation marker. RT-PCR and immunostaining revealed induction of additional lens epithelial or fiber cell differentiation markers including Foxe3, Prox1, α- and β-crystallins, and Tdrd7. Moreover, γA-crystallin- or Prox1-expressing lentoid bodies formed by 30 days in culture. In hES cells, Pax6 or Six3 lentiviral vectors also induced lens marker expression. mES cells that express lens markers reside close to but are distinct from the Pax6 or Six3 transduced cells, suggesting that the latter induce nearby undifferentiated ES cells to adopt a lens fate by non-cell autonomous mechanisms. In sum, we describe a novel mES cell GFP reporter line that is useful for monitoring induction of lens fate, and demonstrate that Pax6 or Six3 is sufficient to induce ES cells to adopt a lens fate, potentially via non-cell autonomous mechanisms. These findings should facilitate investigations of lens development.

The cellular and molecular mechanisms of vertebrate lens development

The ocular lens is a model system for understanding important aspects of embryonic development, such as cell specification and the spatiotemporally controlled formation of a three-dimensional structure. The lens, which is characterized by transparency, refraction and elasticity, is composed of a bulk mass of fiber cells attached to a sheet of lens epithelium. Although lens induction has been studied for over 100 years, recent findings have revealed a myriad of extracellular signaling pathways and gene regulatory networks, integrated and executed by the transcription factor Pax6, that are required for lens formation in vertebrates. This Review summarizes recent progress in the field, emphasizing the interplay between the diverse regulatory mechanisms employed to form lens progenitor and precursor cells and highlighting novel opportunities to fill gaps in our understanding of lens tissue morphogenesis.

Emerging issues in radiogenic cataracts and cardiovascular disease

In 2011, the International Commission on Radiological Protection issued a statement on tissue reactions (formerly termed non-stochastic or deterministic effects) to recommend lowering the threshold for cataracts and the occupational equivalent dose limit for the crystalline lens of the eye. Furthermore, this statement was the first to list circulatory disease (cardiovascular and cerebrovascular disease) as a health hazard of radiation exposure and to assign its threshold for the heart and brain. These changes have stimulated various discussions and may have impacts on some radiation workers, such as those in the medical sector. This paper considers emerging issues associated with cataracts and cardiovascular disease. For cataracts, topics dealt with herein include (i) the progressive nature, stochastic nature, target cells and trigger events of lens opacification, (ii) roles of lens protein denaturation, oxidative stress, calcium ions, tumor suppressors and DNA repair factors in cataractogenesis, (iii) dose rate effect, radiation weighting factor, and classification systems for cataracts, and (iv) estimation of the lens dose in clinical settings. Topics for cardiovascular disease include experimental animal models, relevant surrogate markers, latency period, target tissues, and roles of inflammation and cellular senescence. Future research needs are also discussed.

Ionizing irradiation not only inactivates clonogenic potential in primary normal human diploid lens epithelial cells but also stimulates cell proliferation in a subset of this population

Over the past century, ionizing radiation has been known to induce cataracts in the crystalline lens of the eye, but its mechanistic underpinnings remain incompletely understood. This study is the first to report the clonogenic survival of irradiated primary normal human lens epithelial cells and stimulation of its proliferation. Here we used two primary normal human cell strains: HLEC1 lens epithelial cells and WI-38 lung fibroblasts. Both strains were diploid, and a replicative lifespan was shorter in HLEC1 cells. The colony formation assay demonstrated that the clonogenic survival of both strains decreases similarly with increasing doses of X-rays. A difference in the survival between two strains was actually insignificant, although HLEC1 cells had the lower plating efficiency. This indicates that the same dose inactivates the same fraction of clonogenic cells in both strains. Intriguingly, irradiation enlarged the size of clonogenic colonies arising from HLEC1 cells in marked contrast to those from WI-38 cells. Such enhanced proliferation of clonogenic HLEC1 cells was significant at ≥2 Gy, and manifested as increments of ≤2.6 population doublings besides sham-irradiated controls. These results suggest that irradiation of HLEC1 cells not only inactivates clonogenic potential but also stimulates proliferation of surviving uniactivated clonogenic cells. Given that the lens is a closed system, the stimulated proliferation of lens epithelial cells may not be a homeostatic mechanism to compensate for their cell loss, but rather should be regarded as abnormal. This is because these findings are consistent with the early in vivo evidence documenting that irradiation induces excessive proliferation of rabbit lens epithelial cells and that suppression of lens epithelial cell divisions inhibits radiation cataractogenesis in frogs and rats. Thus, our in vitro model will be useful to evaluate the excessive proliferation of primary normal human lens epithelial cells that may underlie radiation cataractogenesis, warranting further investigations.

Small-molecule induction promotes corneal epithelial cell differentiation from human induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSCs) offer unique opportunities for developing novel cell-based therapies and disease modeling. In this study, we developed a directed differentiation method for hiPSCs toward corneal epithelial progenitor cells capable of terminal differentiation toward mature corneal epithelial-like cells. In order to improve the efficiency and reproducibility of our method, we replicated signaling cues active during ocular surface ectoderm development with the help of two small-molecule inhibitors in combination with basic fibroblast growth factor (bFGF) in serum-free and feeder-free conditions. First, small-molecule induction downregulated the expression of pluripotency markers while upregulating several transcription factors essential for normal eye development. Second, protein expression of the corneal epithelial progenitor marker p63 was greatly enhanced, with up to 95% of cells being p63 positive after 5 weeks of differentiation. Third, corneal epithelial-like cells were obtained upon further maturation.

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Small-molecule induction directs early stage differentiation

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Subsequent maturation yields homogeneous populations of p63-positive cells

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p63-positive progenitor cells are capable of terminal differentiation

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The serum-free and feeder-free method can be upgraded to fully defined and xeno free

What are the intracellular targets and intratissue target cells for radiation effects?

Exactly a century after Röntgen's discovery of X rays, I entered a university to major in radiological sciences. At that time, I felt that, despite extensive use and indispensable roles of ionizing radiation in medicine and industry, many fascinating questions have yet to be answered concerning its biological mechanisms of action, and thus I decided to get into the field of radiation research. Fifteen years have passed since I started radiobiological studies in 1998, during which time various basic tenets I initially learned in my late teens and early twenties have been challenged by recent observations. Of these, this brief overview particularly focuses on the following five different albeit non mutually exclusive questions: (i) "Is nuclear DNA the only intracellular target for radiation effects?"; (ii) "What is the significance of delayed cell death in clonogenic survival?"; (iii) "Does an irradiated cell become a cancer cell?"; (iv) "Are cataracts tissue reactions?"; and (v) "Why is high-LET radiation biologically effective?".

Stem Cell Isolation from Human Wharton's Jelly: A Study of Their Differentiation Ability into Lens Fiber Cells

Recently, the use of stem cells has expanded into numerous areas including cell therapy. In this study, we investigated the differentiation capacity of human Wharton's jelly stem cells (hWJSCs) into lens fiber cells. Morphological changes and expressions of four crystallin genes (αA, αB, βB1 and βB3) were studied. The bovine vitreous body has been shown to induce expression of crystallin genes in hWJSCs. By using the vitreous as a lens fiber cell inducer, we showed that αB-, βB1- and βB3-crystallin genes expressed in hWJSCs.

Identification and characterization of FGF2-dependent mRNA: microRNA networks during lens fiber cell differentiation

MicroRNAs (miRNAs) and fibroblast growth factor (FGF) signaling regulate a wide range of cellular functions, including cell specification, proliferation, migration, differentiation, and survival. In lens, both these systems control lens fiber cell differentiation; however, a possible link between these processes remains to be examined. Herein, the functional requirement for miRNAs in differentiating lens fiber cells was demonstrated via conditional inactivation of Dicer1 in mouse (Mus musculus) lens. To dissect the miRNA-dependent pathways during lens differentiation, we used a rat (Rattus norvegicus) lens epithelial explant system, induced by FGF2 to differentiate, followed by mRNA and miRNA expression profiling. Transcriptome and miRNome analysis identified extensive FGF2-regulated cellular responses that were both independent and dependent on miRNAs. We identified 131 FGF2-regulated miRNAs. Seventy-six of these miRNAs had at least two in silico predicted and inversely regulated target mRNAs. Genes modulated by the greatest number of FGF-regulated miRNAs include DNA-binding transcription factors Nfib, Nfat5/OREBP, c-Maf, Ets1, and N-Myc. Activated FGF signaling influenced bone morphogenetic factor/transforming growth factor-β, Notch, and Wnt signaling cascades implicated earlier in lens differentiation. Specific miRNA:mRNA interaction networks were predicted for c-Maf, N-Myc, and Nfib (DNA-binding transcription factors); Cnot6, Cpsf6, Dicer1, and Tnrc6b (RNA to miRNA processing); and Ash1l, Med1/PBP, and Kdm5b/Jarid1b/Plu1 (chromatin remodeling). Three miRNAs, including miR-143, miR-155, and miR-301a, down-regulated expression of c-Maf in the 3′-UTR luciferase reporter assays. These present studies demonstrate for the first time global impact of activated FGF signaling in lens cell culture system and predicted novel gene regulatory networks connected by multiple miRNAs that regulate lens differentiation.

Histone posttranslational modifications and cell fate determination: lens induction requires the lysine acetyltransferases CBP and p300

Lens induction is a classical embryologic model to study cell fate determination. It has been proposed earlier that specific changes in core histone modifications accompany the process of cell fate specification and determination. The lysine acetyltransferases CBP and p300 function as principal enzymes that modify core histones to facilitate specific gene expression. Herein, we performed conditional inactivation of both CBP and p300 in the ectodermal cells that give rise to the lens placode. Inactivation of both CBP and p300 resulted in the dramatic discontinuation of all aspects of lens specification and organogenesis, resulting in aphakia. The CBP/p300^−/− ectodermal cells are viable and not prone to apoptosis. These cells showed reduced expression of Six3 and Sox2, while expression of Pax6 was not upregulated, indicating discontinuation of lens induction. Consequently, expression of αB- and αA-crystallins was not initiated. Mutant ectoderm exhibited markedly reduced levels of histone H3 K18 and K27 acetylation, subtly increased H3 K27me3 and unaltered overall levels of H3 K9ac and H3 K4me3. Our data demonstrate that CBP and p300 are required to establish lens cell-type identity during lens induction, and suggest that posttranslational histone modifications are integral to normal cell fate determination in the mammalian lens.

Signaling pathways regulating ectodermal cell fate choices

Although embryonic patterning and early development of the nervous system have been studied for decades, our understanding of how signals instruct ectodermal derivatives to acquire specific identities has only recently started to form a coherent picture. In this mini-review, we summarize recent findings and models of how a handful of well-known secreted signals influence progenitor cells in successive binary decisions to adopt various cell type specific differentiation programs.

Cell signaling pathways in vertebrate lens regeneration

Certain vertebrates are capable of regenerating parts of the eye, including the lens. Depending on the species, two principal forms of in vivo lens regeneration have been described wherein the new lens arises from either the pigmented epithelium of the dorsal iris or the cornea epithelium. These forms of lens regeneration are triggered by retinal factors present in the eye. Studies have begun to illuminate the nature of the signals that support lens regeneration. This review describes evidence for the involvement of specific signaling pathways in lens regeneration, including the FGF, retinoic acid, TGF-beta, Wnt, and Hedgehog pathways.

Intrinsic lens forming potential of mouse lens epithelial versus newt iris pigment epithelial cells in three-dimensional culture

Adult newts (Notophthalmus viridescens) are capable of complete lens regeneration that is mediated through dorsal iris pigment epithelial (IPE) cells transdifferentiation. In contrast, higher vertebrates such as mice demonstrate only limited lens regeneration in the presence of an intact lens capsule with remaining lens epithelial cells. To compare the intrinsic lens regeneration potential of newt IPE versus mouse lens epithelial cells (MLE), we have established a novel culture method that uses cell aggregation before culture in growth factor-reduced Matrigel. Dorsal newt IPE aggregates demonstrated complete lens formation within 1 to 2 weeks of Matrigel culture without basic fibroblast growth factor (bFGF) supplementation, including the establishment of a peripheral cuboidal epithelial cell layer, and the appearance of central lens fibers that were positive for αA-crystallin. In contrast, the lens-forming potential of MLE cell aggregates cultured in Matrigel was incomplete and resulted in the formation of defined-size lentoids with partial optical transparency. While the peripheral cell layers of MLE aggregates were nucleated, cells in the center of aggregates demonstrated a nonapoptotic nuclear loss over a time period of 3 weeks that was representative of lens fiber formation. Matrigel culture supplementation with bFGF resulted in higher transparent bigger-size MLE aggregates that demonstrated increased appearance of βB1-crystallin expression. Our study demonstrates that bFGF is not required for induction of newt IPE aggregate-dependent lens formation in Matrigel, while the addition of bFGF seems to be beneficial for the formation of MLE aggregate-derived lens-like structures. In conclusion, the three-dimensional aggregate culture of IPE and MLE in Matrigel allows to a higher extent than older models the indepth study of the intrinsic lens-forming potential and the corresponding identification of lentogenic factors.

Differential BMP signaling controls formation and differentiation of multipotent preplacodal ectoderm progenitors from human embryonic stem cells

Sensory and endoneurocrine tissues as diverse as the lens, the olfactory epithelium, the inner ear, the cranial sensory ganglia, and the anterior pituitary arise from a common pool of progenitors in the preplacodal ectoderm (PPE). Around late gastrulation, the PPE forms at the border surrounding the anterior neural plate, and expresses a unique set of evolutionarily conserved transcription regulators including Six1, Eya 1 and Eya2. Here, we describe the first report to generate and characterize the SIX1(+) PPE cells from human embryonic stem (ES) cells by adherent differentiation. Before forming PPE cells, differentiating cultures first expressed the non-neural ectoderm specific transcriptional factors TFAP2A, GATA2, GATA3, DLX3, and DLX5, which are crucial in establishing the PPE competence. We demonstrated that bone morphogenetic protein (BMP) activity plays a transient but essential role in inducing expression of these PPE competence factors and eventually the PPE cells. Interestingly, we found that attenuating BMP signaling after establishing the competence state induces anterior placode precursors. By manipulating BMP and hedgehog signaling pathways, we further differentiate these precursors into restricted lineages including the lens placode and the oral ectoderm (pituitary precursor) cells. Finally, we also show that sensory neurons can be generated from human PPE cells, demonstrating the multipotency of the human ES-derived PPE cells.

The 3R principle: advancing clinical application of human pluripotent stem cells

The first derivation of human embryonic stem cells brought with it a clear understanding that animal models of human disease might be replaced by an unlimited supply of human cells for research, drug discovery, and drug development. With the advent of clinical trials using human pluripotent stem cell-based therapies, it is both timely and relevant to reflect on factors that will facilitate future translation of this technology. Human pluripotent cells are increasingly being used to investigate the molecular mechanisms that underpin normal and pathological human development. Their differentiated progeny are also being used to identify novel pharmaceuticals, to screen for toxic effects of known chemicals, and to investigate cell or tissue transplantation strategies. The intrinsic assumption of these research efforts is that the information gained from these studies will be more accurate, and therefore of greater relevance, than traditional investigations based on animal models of human disease and injury. This review will therefore evaluate how animals and animal-derived products are used for human pluripotent stem cell research, and will indicate how efforts to further reduce or remove animals and animal products from this research will increase the clinical translation of human pluripotent stem cell technologies through drug discovery, toxicology screening, and cell replacement therapies.

Stem Cells and Ocular Tissue Regeneration

PURPOSE

Millions worldwide have visual impairments caused by dysfunctional eye components, including cornea, lens, retina, and optic nerve, or the visual cortex in the brain. Insufficient cornea donation and inherent artificial lens problems demand alternative treatment strategies for cornea diseases and cataracts, whereas retinal degenerative diseases, including glaucoma, macular degeneration, and retinitis pigmentosa, still lack effective treatments. Stem cells have been investigated for their potential in various eye-specific pathologies to replace lost retinal ganglion cells and photoreceptors in retinal degenerative diseases and toward engineering transplantable patient-specific cornea or lens.

DESIGN

Many stem cell types, including putative resident eye stem cells, mesenchymal stem cells, embryonic stem cells, and induced pluripotent stem cells, have been investigated for their potential to generate specific cell types in the eye in culture and after transplantation and to engineer eye tissues in combination with structural scaffolds.

METHOD

Cultured stem cells and in vitro differentiated eye-specific cells are transplanted into different locations of the eye to test their ability to produce functional cells for supporting eye functions. In addition, stem cells have been directly tested in vitro for their capacity to engineer eye-specific tissues.

RESULTS

Different stem cell types have been shown to have distinct capacities to produce eye-specific cells or even the entire retina.

CONCLUSIONS

Stem cells offer great hope for treating various eye pathologies. Despite recent progress, many challenges must still be overcome before the era of stem cell-based therapy in the eye truly arrives.

Derivation of multiple cranial tissues and isolation of lens epithelium-like cells from human embryonic stem cells

Human embryonic stem cells (hESCs) provide a powerful tool to investigate early events occurring during human embryonic development. In the present study, we induced differentiation of hESCs in conditions that allowed formation of neural and non-neural ectoderm and to a lesser extent mesoderm. These tissues are required for correct specification of the neural plate border, an early embryonic transient structure from which neural crest cells (NCs) and cranial placodes (CPs) originate. Although isolation of CP derivatives from hESCs has not been previously reported, isolation of hESC-derived NC-like cells has been already described. We performed a more detailed analysis of fluorescence-activated cell sorting (FACS)-purified cell populations using the surface antigens previously used to select hESC-derived NC-like cells, p75 and HNK-1, and uncovered their heterogeneous nature. In addition to the NC component, we identified a neural component within these populations using known surface markers, such as CD15 and FORSE1. We have further exploited this information to facilitate the isolation and purification by FACS of a CP derivative, the lens, from differentiating hESCs. Two surface markers expressed on lens cells, c-Met/HGFR and CD44, were used for positive selection of multiple populations with a simultaneous subtraction of the neural/NC component mediated by p75, HNK-1, and CD15. In particular, the c-Met/HGFR allowed early isolation of proliferative lens epithelium-like cells capable of forming lentoid bodies. Isolation of hESC-derived lens cells represents an important step toward the understanding of human lens development and regeneration and the devising of future therapeutic applications.

Primary cultures of embryonic chick lens cells as a model system to study lens gap junctions and fiber cell differentiation

A major limitation in lens gap junction research has been the lack of experimentally tractable ex vivo systems to study the formation and regulation of fiber-type gap junctions. Although immortalized lens-derived cell lines are amenable to both gene transfection and siRNA-mediated knockdown, to our knowledge none are capable of undergoing appreciable epithelial-to-fiber differentiation. Lens central epithelial explants have the converse limitation. A key advance in the field was the development of a primary embryonic chick lens cell culture system by Drs. Sue Menko and Ross Johnson. Unlike central epithelial explants, these cultures also include cells from the peripheral (preequatorial and equatorial) epithelium, which is the most physiologically relevant population for the study of fiber-type gap junction formation. We have modified the Menko/Johnson system and refer to our cultures as dissociated cell-derived monolayer cultures (DCDMLs). We culture DCDMLs without serum to mimic the avascular lens environment and on laminin, the major matrix component of the lens capsule. Here, I review the features of the DCDML system and how we have used it to study lens gap junctions and fiber cell differentiation. Our results demonstrate the power of DCDMLs to generate new findings germane to the mammalian lens and how these cultures can be exploited to conduct experiments that would be impossible, prohibitively expensive and/or difficult to interpret using transgenic animals in vivo.

A balance of FGF and BMP signals regulates cell cycle exit and Equarin expression in lens cells

The roles of BMP and FGF during the transition of proliferating lens epithelial cells to differentiated primary lens fiber cells are examined. The results show that proliferation, cell cycle exit, and early differentiation of primary lens fiber cells are regulated by counterbalancing BMP and FGF signals.

In embryonic and adult lenses, a balance of cell proliferation, cell cycle exit, and differentiation is necessary to maintain physical function. The molecular mechanisms regulating the transition of proliferating lens epithelial cells to differentiated primary lens fiber cells are poorly characterized. To investigate this question, we used gain- and loss-of-function analyses to modulate fibroblast growth factor (FGF) and/or bone morphogenetic protein (BMP) signals in chick lens/retina explants. Here we show that FGF activity plays a key role for proliferation independent of BMP signals. Moreover, a balance of FGF and BMP signals regulates cell cycle exit and the expression of Ccdc80 (also called Equarin), which is expressed at sites where differentiation of lens fiber cells occurs. BMP activity promotes cell cycle exit and induces Equarin expression in an FGF-dependent manner. In contrast, FGF activity is required but not sufficient to induce cell cycle exit or Equarin expression. Furthermore, our results show that in the absence of BMP activity, lens cells have increased cell cycle length or are arrested in the cell cycle, which leads to decreased cell cycle exit. Taken together, these findings suggest that proliferation, cell cycle exit, and early differentiation of primary lens fiber cells are regulated by counterbalancing BMP and FGF signals.

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.

Efficient generation of lens progenitor cells from cataract patient-specific induced pluripotent stem cells

The development of a technique to induce the transformation of somatic cells to a pluripotent state via the ectopic expression of defined transcription factors was a transformational event in the field of regenerative medicine. The development of this technique also impacted ophthalmology, as patient-specific induced pluripotent stemcells (iPSCs) may be useful resources for some ophthalmological diseases. The lens is a key refractive element in the eye that focuses images of the visual world onto the retina. To establish a new model for drug screening to treat lens diseases and investigating lens aging and development, we examined whether human lens epithelial cells (HLECs) could be induced into iPSCs and if lens-specific differentiation of these cells could be achieved under defined chemical conditions. We first efficiently reprogrammed HLECs from age-related cataract patients to iPSCs with OCT-4, SOX-2, and KLF-4. The resulting HLEC-derived iPS (HLE-iPS) colonies were indistinguishable from human ES cells with respect to morphology, gene expression, pluripotent marker expression and their ability to generate all embryonic germ-cell layers. Next, we performed a 3-step induction procedure: HLE-iPS cells were differentiated into large numbers of lens progenitor-like cells with defined factors (Noggin, BMP and FGF2), and we determined that these cells expressed lens-specific markers (PAX6, SOX2, SIX3, CRYAB, CRYAA, BFSP1, and MIP). In addition, HLE-iPS-derived lens cells exhibited reduced expression of epithelial mesenchymal transition (EMT) markers compared with human embryonic stem cells (hESCs) and fibroblast-derived iPSCs. Our study describes a highly efficient procedure for generating lens progenitor cells from cataract patient HLEC-derived iPSCs. These patient-derived pluripotent cells provide a valuable model for studying the developmental and molecular biological mechanisms that underlie cell determination in lens development and cataract pathophysiology.

The orchestration of mammalian tissue morphogenesis through a series of coherent feed-forward loops

Tissue morphogenesis requires intricate temporal and spatial control of gene expression that is executed through specific gene regulatory networks (GRNs). GRNs are comprised from individual subcircuits of different levels of complexity. An important question is to elucidate the mutual relationship between those genes encoding DNA-binding factors that trigger the subcircuit with those that play major "later" roles during terminal differentiation via expression of specific genes that constitute the phenotype of individual tissues. The ocular lens is a classical model system to study tissue morphogenesis. Pax6 is essential for both lens placode formation and subsequent stages of lens morphogenesis, whereas c-Maf controls terminal differentiation of lens fibers, including regulation of crystallins, key lens structural proteins required for its transparency and refraction. Here, we show that Pax6 directly regulates c-Maf expression during lens development. A 1.3-kb c-Maf promoter with a 1.6-kb upstream enhancer (CR1) recapitulated the endogenous c-Maf expression pattern in lens and retinal pigmented epithelium. ChIP assays revealed binding of Pax6 and c-Maf to multiple regions of the c-Maf locus in lens chromatin. To predict functional Pax6-binding sites, nine novel variants of Pax6 DNA-binding motifs were identified and characterized. Two of these motifs predicted a pair of Pax6-binding sites in the CR1. Mutagenesis of these Pax6-binding sites inactivated transgenic expression in the lens but not in retinal pigmented epithelium. These data establish a novel regulatory role for Pax6 during lens development, link together the Pax6/c-Maf/crystallin regulatory network, and suggest a novel type of GRN subcircuit that controls a major part of embryonic lens development.

Lens fibre cell differentiation and organelle loss: many paths lead to clarity

The programmed removal of organelles from differentiating lens fibre cells contributes towards lens transparency through formation of an organelle-free zone (OFZ). Disruptions in OFZ formation are accompanied by the persistence of organelles in lens fibre cells and can contribute towards cataract. A great deal of work has gone into elucidating the nature of the mechanisms and signalling pathways involved. It is apparent that multiple, parallel and redundant pathways are involved in this process and that these pathways form interacting networks. Furthermore, it is possible that the pathways can functionally compensate for each other, for example in mouse knockout studies. This makes sense given the importance of lens clarity in an evolutionary context. Apoptosis signalling and proteolytic pathways have been implicated in both lens fibre cell differentiation and organelle loss, including the Bcl-2 and inhibitor of apoptosis families, tumour necrosis factors, p53 and its regulators (such as Mdm2) and proteolytic enzymes, including caspases, cathepsins, calpains and the ubiquitin-proteasome pathway. Ongoing approaches being used to dissect the molecular pathways involved, such as transgenics, lens-specific gene deletion and zebrafish mutants, are discussed here. Finally, some of the remaining unresolved issues and potential areas for future studies are highlighted.