Acidic and basic FGF in ocular media and lens: implications for lens polarity and growth patterns

A Transcriptomics Analysis of the Regulation of Lens Fiber Cell Differentiation in the Absence of FGFRs and PTEN

Adding 50% vitreous humor to the media surrounding lens explants induces fiber cell differentiation and a significant immune/inflammatory response. While Fgfr loss blocks differentiation in lens epithelial explants, this blockage is partially reversed by deleting Pten. To investigate the functions of the Fgfrs and Pten during lens fiber cell differentiation, we utilized a lens epithelial explant system and conducted RNA sequencing on vitreous humor-exposed explants lacking Fgfrs, or Pten or both Fgfrs and Pten. We found that Fgfr loss impairs both vitreous-induced differentiation and inflammation while the additional loss of Pten restores these responses. Furthermore, transcriptomic analysis suggested that PDGFR-signaling in FGFR-deficient explants is required to mediate the rescue of vitreous-induced fiber differentiation in explants lacking both Fgfrs and Pten. The blockage of β-crystallin induction in explants lacking both Fgfrs and Pten in the presence of a PDGFR inhibitor supports this hypothesis. Our findings demonstrate that a wide array of genes associated with fiber cell differentiation are downstream of FGFR-signaling and that the vitreous-induced immune responses also depend on FGFR-signaling. Our data also demonstrate that many of the vitreous-induced gene-expression changes in Fgfr-deficient explants are rescued in explants lacking both Fgfrs and Pten.

TGFβ overcomes FGF-induced transinhibition of EGFR in lens cells to enable fibrotic secondary cataract

To cause vision-disrupting fibrotic secondary cataract (PCO), lens epithelial cells that survive cataract surgery must migrate to the posterior of the lens capsule and differentiate into myofibroblasts. During this process, the cells become exposed to the FGF that diffuses out of the vitreous body. In normal development, such relatively high levels of FGF induce lens epithelial cells to differentiate into lens fiber cells. It has been a mystery as to how lens cells could instead undergo a mutually exclusive cell fate, namely epithelial to myofibroblast transition, in the FGF-rich environment of the posterior capsule. We and others have reported that the ability of TGFβ to induce lens cell fibrosis requires the activity of endogenous ErbBs. We show here that lens fiber-promoting levels of FGF induce desensitization of ErbB1 (EGFR) that involves its phosphorylation on threonine 669 mediated by both ERK and p38 activity. Transinhibition of ErbB1 by FGF is overcome by a time-dependent increase in ErbB1 levels induced by TGFβ, the activation of which is increased after cataract surgery. Our studies provide a rationale for why TGFβ upregulates ErbB1 in lens cells and further support the receptor as a therapeutic target for PCO.

Fibroblast growth factor-induced lens fiber cell elongation is driven by the stepwise activity of Rho and Rac

The spheroidal shape of the eye lens is crucial for precise light focusing onto the retina. This shape is determined by concentrically aligned, convexly elongated lens fiber cells along the anterior and posterior axis of the lens. Upon differentiation at the lens equator, the fiber cells increase in height as their apical and basal tips migrate towards the anterior and posterior poles, respectively. The forces driving this elongation and migration remain unclear. We found that, in the mouse lens, membrane protrusions or lamellipodia are observed only in the maturing fibers undergoing cell curve conversion, indicating that lamellipodium formation is not the primary driver of earlier fiber migration. We demonstrated that elevated levels of fibroblast growth factor (FGF) suppressed the extension of Rac-dependent protrusions, suggesting changes in the activity of FGF controlling Rac activity, switching to lamellipodium-driven migration. Inhibitors of ROCK, myosin and actin reduced the height of both early and later fibers, indicating that elongation of these fibers relies on actomyosin contractility. Consistent with this, active RhoA was detected throughout these fibers. Given that FGF promotes fiber elongation, we propose that it does so through regulation of Rho activity.

Fibroblast Growth Factor-induced lens fiber cell elongation is driven by the stepwise activity of Rho and Rac

The spheroidal shape of the eye lens is critical for precise light focusing onto the retina. This shape is determined by concentrically aligned, convexly elongated lens fiber cells along the anterior and posterior axis of the lens. Upon differentiation at the lens equator, the fiber cells increase in height as their apical and basal tips migrate towards the anterior and posterior poles, respectively. The forces driving this elongation and migration remain unclear. We found that membrane protrusions or lamellipodia are observed only in the maturing fibers undergoing cell curve conversion, indicating lamellipodium is not the primary driver of earlier fiber migration. We demonstrated that elevated levels of fibroblast growth factor (FGF) suppressed the extension of Rac-dependent protrusions, suggesting changes in the activity of FGF controling Rac activity, switching to lamellipodium-driven migration. Inhibitors of ROCK, myosin, and actin reduced the height of both early and later fibers, indicating elongation of these fibers relies on actomyosin contractility. Consistently, active RhoA was detected throughout these fibers. Given that FGF promotes fiber elongation, we propose it to do so through regulation of Rho activity.

Multiomics Analysis Reveals Novel Genetic Determinants for Lens Differentiation, Structure, and Transparency

Recent advances in next-generation sequencing and data analysis have provided new gateways for identification of novel genome-wide genetic determinants governing tissue development and disease. These advances have revolutionized our understanding of cellular differentiation, homeostasis, and specialized function in multiple tissues. Bioinformatic and functional analysis of these genetic determinants and the pathways they regulate have provided a novel basis for the design of functional experiments to answer a wide range of long-sought biological questions. A well-characterized model for the application of these emerging technologies is the development and differentiation of the ocular lens and how individual pathways regulate lens morphogenesis, gene expression, transparency, and refraction. Recent applications of next-generation sequencing analysis on well-characterized chicken and mouse lens differentiation models using a variety of omics techniques including RNA-seq, ATAC-seq, whole-genome bisulfite sequencing (WGBS), chip-seq, and CUT&RUN have revealed a wide range of essential biological pathways and chromatin features governing lens structure and function. Multiomics integration of these data has established new gene functions and cellular processes essential for lens formation, homeostasis, and transparency including the identification of novel transcription control pathways, autophagy remodeling pathways, and signal transduction pathways, among others. This review summarizes recent omics technologies applied to the lens, methods for integrating multiomics data, and how these recent technologies have advanced our understanding ocular biology and function. The approach and analysis are relevant to identifying the features and functional requirements of more complex tissues and disease states.

Heparan sulfate proteoglycans (HSPGs) of the ocular lens

Heparan sulfate proteoglycans (HSPGs) reside in most cells; on their surface, in the pericellular milieu and/or extracellular matrix. In the eye, HSPGs can orchestrate the activity of key signalling molecules found in the ocular environment that promote its development and homeostasis. To date, our understanding of the specific roles played by individual HSPG family members, and the heterogeneity of their associated sulfated HS chains, is in its infancy. The crystalline lens is a relatively simple and well characterised ocular tissue that provides an ideal stage to showcase and model the expression and unique roles of individual HSPGs. Individual HSPG core proteins are differentially localised to eye tissues in a temporal and spatial developmental- and cell-type specific manner, and their loss or functional disruption results in unique phenotypic outcomes for the lens, and other ocular tissues. More recent work has found that different HS sulfation enzymes are also presented in a cell- and tissue-specific manner, and that disruption of these different sulfation patterns affects specific HS-protein interactions. Not surprisingly, these sulfated HS chains have also been reported to be required for lens and eye development, with dysregulation of HS chain structure and function leading to pathogenesis and eye-related phenotypes. In the lens, HSPGs undergo significant and specific changes in expression and function that can drive pathology, or in some cases, promote tissue repair. As master signalling regulators, HSPGs may one day serve as valuable biomarkers, and even as putative targets for the development of novel therapeutics, not only for the eye but for many other systemic pathologies.

Lens Epithelial Explants Treated with Vitreous Humor Undergo Alterations in Chromatin Landscape with Concurrent Activation of Genes Associated with Fiber Cell Differentiation and Innate Immune Response

Lens epithelial explants are comprised of lens epithelial cells cultured in vitro on their native basement membrane, the lens capsule. Biologists have used lens epithelial explants to study many different cellular processes including lens fiber cell differentiation. In these studies, fiber differentiation is typically measured by cellular elongation and the expression of a few proteins characteristically expressed by lens fiber cells in situ. Chromatin and RNA was collected from lens epithelial explants cultured in either un-supplemented media or media containing 50% bovine vitreous humor for one or five days. Chromatin for ATAC-sequencing and RNA for RNA-sequencing was prepared from explants to assess regions of accessible chromatin and to quantitatively measure gene expression, respectively. Vitreous humor increased chromatin accessibility in promoter regions of genes associated with fiber differentiation and, surprisingly, an immune response, and this was associated with increased transcript levels for these genes. In contrast, vitreous had little effect on the accessibility of the genes highly expressed in the lens epithelium despite dramatic reductions in their mRNA transcripts. An unbiased analysis of differentially accessible regions revealed an enrichment of cis-regulatory motifs for RUNX, SOX and TEAD transcription factors that may drive differential gene expression in response to vitreous.

The lens epithelium as a major determinant in the development, maintenance, and regeneration of the crystalline lens

The crystalline lens is a transparent and refractive biconvex structure formed by lens epithelial cells (LECs) and lens fibers. Lens opacity, also known as cataracts, is the leading cause of blindness in the world. LECs are the principal cells of lens throughout human life, exhibiting different physiological properties and functions. During the embryonic stage, LECs proliferate and differentiate into lens fibers, which form the crystalline lens. Genetics and environment are vital factors that influence normal lens development. During maturation, LECs help maintain lens homeostasis through material transport, synthesis and metabolism as well as mitosis and proliferation. If disturbed, this will result in loss of lens transparency. After cataract surgery, the repair potential of LECs is activated and the structure and transparency of the regenerative tissue depends on postoperative microenvironment. This review summarizes recent research advances on the role of LECs in lens development, homeostasis, and regeneration, with a particular focus on the role of cholesterol synthesis (eg., lanosterol synthase) in lens development and homeostasis maintenance, and how the regenerative potential of LECs can be harnessed to develop surgical strategies and improve the outcomes of cataract surgery (Fig. 1). These new insights suggest that LECs are a major determinant of the physiological and pathological state of the lens. Further studies on their molecular biology will offer possibility to explore new approaches for cataract prevention and treatment.

Generation of Lens Progenitor Cells and Lentoid Bodies from Pluripotent Stem Cells: Novel Tools for Human Lens Development and Ocular Disease Etiology

In vitro differentiation of human pluripotent stem cells (hPSCs) into specialized tissues and organs represents a powerful approach to gain insight into those cellular and molecular mechanisms regulating human development. Although normal embryonic eye development is a complex process, generation of ocular organoids and specific ocular tissues from pluripotent stem cells has provided invaluable insights into the formation of lineage-committed progenitor cell populations, signal transduction pathways, and self-organization principles. This review provides a comprehensive summary of recent advances in generation of adenohypophyseal, olfactory, and lens placodes, lens progenitor cells and three-dimensional (3D) primitive lenses, "lentoid bodies", and "micro-lenses". These cells are produced alone or "community-grown" with other ocular tissues. Lentoid bodies/micro-lenses generated from human patients carrying mutations in crystallin genes demonstrate proof-of-principle that these cells are suitable for mechanistic studies of cataractogenesis. Taken together, current and emerging advanced in vitro differentiation methods pave the road to understand molecular mechanisms of cataract formation caused by the entire spectrum of mutations in DNA-binding regulatory genes, such as PAX6, SOX2, FOXE3, MAF, PITX3, and HSF4, individual crystallins, and other genes such as BFSP1, BFSP2, EPHA2, GJA3, GJA8, LIM2, MIP, and TDRD7 represented in human cataract patients.

MYPT1/PP1-Mediated EZH2 Dephosphorylation at S21 Promotes Epithelial-Mesenchymal Transition in Fibrosis through Control of Multiple Families of Genes

The methyltransferase EZH2 plays an important role in regulating chromatin conformation and gene transcription. Phosphorylation of EZH2 at S21 by AKT kinase suppresses its function. However, protein phosphatases responsible for the dephosphorylation of EZH2-S21 remain elusive. Here, it is demonstrated that EZH2 is highly expressed in the ocular lens, and AKT-EZH2 axis is important in TGFβ-induced epithelial-mesenchymal transition (EMT). More importantly, it is identified that MYPT1/PP1 dephosphorylates EZH2-S21 and thus modulates its functions. MYPT1 knockout accelerates EMT, but expression of the EZH2-S21A mutant suppresses EMT through control of multiple families of genes. Furthermore, the phosphorylation status and gene expression modulation of EZH2 are implicated in control of anterior subcapsular cataracts (ASC) in human and mouse eyes. Together, the results identify the specific phosphatase for EZH2-S21 and reveal EZH2 dephosphorylation control of several families of genes implicated in lens EMT and ASC pathogenesis. These results provide important novel information in EZH2 function and regulation.

An Atlas of Heparan Sulfate Proteoglycans in the Postnatal Rat Lens

Purpose

The arrangement of lens cells is regulated by ocular growth factors. Although the effects of these inductive molecules on lens cell behavior (proliferation, survival, and fiber differentiation) are well-characterized, the precise mechanisms underlying the regulation of growth factor-mediated signaling in lens remains elusive. Increasing evidence highlights the importance of heparan sulfate proteoglycans (HSPGs) for the signaling regulation of growth factors; however, the identity of the different lens HSPGs and the specific roles they play in lens biology are still unknown.

Methods

Semiquantitative real-time (RT)‐PCR and immunolabeling were used to characterize the spatial distribution of all known HSPG core proteins and their associated glycosaminoglycans (heparan and chondroitin sulfate) in the postnatal rat lens. Fibroblast growth factor (FGF)-2-treated lens epithelial explants, cultured in the presence of Surfen (an inhibitor of heparan sulfate [HS]-growth factor binding interactions) were used to investigate the requirement for HS in FGF-2-induced proliferation, fiber differentiation, and ERK1/2-signaling.

Results

The lens expresses all HSPGs. These HSPGs are differentially localized to distinct functional regions of the lens. In vitro, inhibition of HS-sulfation with Surfen blocked FGF-2-mediated ERK1/2-signaling associated with lens epithelial cell proliferation and fiber differentiation, highlighting that these cellular processes are dependent on HS.

Conclusions

These findings support a requirement for HSPGs in FGF-2 driven lens cell proliferation and fiber differentiation. The identification of specific HSPG core proteins in key functional lens regions, and the divergent expression patterns of closely related HSPGs, suggests that different HSPGs may differentially regulate growth factor signaling networks leading to specific biological events involved in lens growth and maintenance.

Lens regeneration: scientific discoveries and clinical possibilities

In the process of exploring new methods for cataract treatment, lens regeneration is an ideal strategy for effectively restoring accommodative vision and avoiding postoperative complications and has great clinical potential. Lens regeneration, which is not a simple repetition of lens development, depends on the complex regulatory network comprising the FGF, BMP/TGF-β, Notch, and Wnt signaling pathways. Current research mainly focuses on in situ and in vitro lens regeneration. On the one hand, the possibility of the autologous stem cell in situ regeneration of functional lenses has been confirmed; on the other hand, both embryonic stem cells and induced pluripotent stem cells have been induced into lentoid bodies in vitro which are similar to the natural lens to a certain extent. This article will briefly summarize the regulatory mechanisms of lens development, describe the recent progress of lens regeneration, explore the key molecular signaling pathways, and, more importantly, discuss the prospects and challenges of their clinical applications to provide reference for clinical transformations.

Lens fiber cell differentiation occurs independently of fibroblast growth factor receptor signaling in the absence of Pten

Fibroblast growth factor receptor (FGFR) signaling patterns multiple tissues in both vertebrates and invertebrates, largely through the activation of intracellular kinases. Recent studies have demonstrated that the phosphatase, PTEN negatively regulates FGFR signaling, such that the loss of PTEN can compensate for reduced FGFR signaling to rescue aspects of normal development. In the developing mouse lens, FGFR signaling promotes cell survival and fiber cell differentiation, and the loss of Pten largely compensates for the loss of Fgfr2 during lens development. To explore this regulatory relationship further, we focused on the phenotypic consequences of Pten loss on lens development and fiber cell differentiation in the absence of all FGFR signaling, both in vivo and in lens epithelial explants. Pten deletion partially rescues primary fiber cell elongation and γ-crystallin accumulation in FGFR-deficient lenses in vivo but fails to rescue cell survival or proliferation. However, in lens epithelial explants, where cells survive without FGFR signaling, Pten deletion rescues vitreous humor-induced lens fiber cell differentiation in the combined absence of Fgfr1, Fgfr2 and Fgfr3. This represents the first evidence that vitreous-initiated signaling cascades, independent of FGFR signaling, can drive mammalian lens fiber cell differentiation, when freed from repression by PTEN.

A fine-tuned β-catenin regulation during proliferation of corneal endothelial cells revealed using proteomics analysis

Corneal endothelial (CE) dysfunction is the main indication for corneal transplantation, an invasive procedure with several limitations. Developing novel strategies to re-activate CE regenerative capacity is, therefore, of fundamental importance. This goal has proved to be challenging as corneal endothelial cells (CEnC) are blocked in the G0/G1 phase of the cell cycle in vivo and, albeit retaining proliferative capacity in vitro, this is further hindered by endothelial-to-mesenchymal transition. Herein we investigated the mechanisms regulating CEnC proliferation in vitro. Comparing the proteome of non-proliferating (in vivo-G0/G1) and proliferating (in vitro-G2/M) rabbit CEnC (rCEnC), 77 proteins, out of 3,328 identified, were differentially expressed in the two groups (p < 0.005). Literature and Gene Ontology analysis revealed β-catenin and transforming growth factor (TGF-β) pathways to be correlated with the identified proteins. Treatment of rCEnC with a β-catenin activator and inhibitor showed that β-catenin activation was necessary during rCEnC proliferation, but not sufficient for its induction. Furthermore, both pro-proliferative activity of basic fibroblast growth factor and anti-proliferative effects of TGF-β were regulated through β-catenin. Overall, these results provide novel insights into the molecular basis underlying the proliferation process that CEnC re-activate in vitro, consolidating the role of β-catenin and TGF-β.

Spred negatively regulates lens growth by modulating epithelial cell proliferation and fiber differentiation

Spred, like Sprouty (Spry) and also Sef proteins, have been identified as important regulators of receptor tyrosine kinase (RTK)-mediated MAPK/ERK-signaling in various developmental systems, controlling cellular processes such as proliferation, migration and differentiation. Spreds are widely expressed during early embryogenesis, and in the eye lens, become more localised in the lens epithelium with later development, overlapping with other antagonists including Spry. Given the synexpression of Spreds and Spry in lens, in order to gain a better understanding of their specific roles in regulating growth factor mediated-signaling and cell behavior, we established and characterised lines of transgenic mice overexpressing Spred1 or Spred2, specifically in the lens. This overexpression of Spreds resulted in a small lens phenotype during ocular morphogenesis, retarding its growth by compromising epithelial cell proliferation and fiber differentiation. These in situ findings were shown to be dependent on the ability of Spreds to suppress MAPK-signaling, in particular FGF-induced ERK1/2-signaling in lens cells. This was validated in vitro using lens epithelial explants, that highlighted the overlapping role of Spreds with Spry2, but not Spry1. This study provides insights into the putative function of Spreds and Spry in situ, some overlapping and some distinct, and their importance in regulating lens cell proliferation and fiber differentiation contributing to lens and eye growth.

Aqueous humour-induced lens epithelial cell proliferation requires FGF-signalling

The eye lens grows by systematic proliferation of its epithelial cells and their differentiation into fibre cells. The anterior aqueous humour regulates lens epithelial cell proliferation whereas posteriorly, the vitreous stimulates lens fibre differentiation. Vitreous-derived members of the fibroblast growth factor (FGF) family induce fibre differentiation, with added support for FGFs as putative regulators of aqueous-induced lens cell proliferation. To further characterize this, given FGFs' known affinity for proteoglycans, we compared the effect of proteoglycan sulphation in growth factor- and aqueous-induced lens cell proliferation. Disruption of proteoglycan sulphation in lens cells specifically impacted on aqueous- and FGF-induced MAPK/ERK1/2-signalling, but not on that induced by other mitogens such as PDGF; however, cell proliferation was reduced in all treatment groups, regardless of the mitogen. Overall, by disrupting proteoglycan activity, we further highlight the significant role of FGFs in aqueous-induced ERK1/2 phosphorylation leading to lens cell proliferation.

Immunohistochemical and western blot analysis suggest that the soluble forms of FGF1-2 and FGFR1-2 sustain tail regeneration in the lizard

Fibroblast Growth Factors 1-2 (FGF1-2) stimulate tail regeneration in lizards and therefore the distribution of their receptors, FGFR1-2, in the regenerating tail of the lizard. Podarcis muralis has been studied using immunofluorescence and western blotting. Immunoreactive protein bands at 15-16kDa for FGF1-2 in addition to those at 50-65kDa are detected in the regenerating epidermis, but weak bands at 35, 45 and 50kDa appear from the regenerating connective tissues. Strongly immunolabeled bands for FGFR1 at 32, 60, and 80kDa and less intense for FGFR2 only appear in the regenerating tail. In normal tail epidermis and dermis, higher MW forms are present at 80 and 115-140kDa, respectively, but they disappear in the regenerating epidermis and dermis where low MW forms of FGFR1-2 are found at 50-70kDa. Immunolocalization confirms that most FGFR1-2 are present in the wound epidermis, Apical Epidermal Peg, ependymal tube while immunolabeling lowers in regenerating muscles, blastema cells, cartilage and connectives tissues. The likely release of FGFs from the Apical Epidermal Peg and ependyma and the presence of their receptors in these tissues may determine the autocrine stimulation of proliferation and a paracrine stimulation of the blastema cells through their FGF Receptors.

The lens growth process

The factors that regulate the size of organs to ensure that they fit within an organism are not well understood. A simple organ, the ocular lens serves as a useful model with which to tackle this problem. In many systems, considerable variance in the organ growth process is tolerable. This is almost certainly not the case in the lens, which in addition to fitting comfortably within the eyeball, must also be of the correct size and shape to focus light sharply onto the retina. Furthermore, the lens does not perform its optical function in isolation. Its growth, which continues throughout life, must therefore be coordinated with that of other tissues in the optical train. Here, we review the lens growth process in detail, from pioneering clinical investigations in the late nineteenth century to insights gleaned more recently in the course of cell and molecular studies. During embryonic development, the lens forms from an invagination of surface ectoderm. Consequently, the progenitor cell population is located at its surface and differentiated cells are confined to the interior. The interactions that regulate cell fate thus occur within the obligate ellipsoidal geometry of the lens. In this context, mathematical models are particularly appropriate tools with which to examine the growth process. In addition to identifying key growth determinants, such models constitute a framework for integrating cell biological and optical data, helping clarify the relationship between gene expression in the lens and image quality at the retinal plane.

Secreted Protein Acidic and Rich in Cysteine in Ocular Tissue

Secreted protein acidic and rich in cysteine (SPARC), also known as osteonectin or BM-40, is the prototypical matricellular protein. Matricellular proteins are nonstructural secreted proteins that provide an integration between cells and their surrounding extracellular matrix (ECM). Regulation of the ECM is important in maintaining the physiologic function of tissues. Elevated levels of SPARC have been identified in a variety of diseases involving pathologic tissue remodeling, such as hepatic fibrosis, systemic sclerosis, and certain carcinomas. Within the eye, SPARC has been identified in the trabecular meshwork, lens, and retina. Studies have begun to show the role of SPARC in these tissues and its possible role, specifically in primary open-angle glaucoma, cataracts, and proliferative vitreoretinopathy. SPARC may, therefore, be a therapeutic target in the treatment of certain ocular diseases. Further investigation into the mechanism of action of SPARC will be necessary in the development of SPARC-targeted therapy.

Synergistic interaction between the fibroblast growth factor and bone morphogenetic protein signaling pathways in lens cells

Relatively little is known about how receptor tyrosine kinase ligands can positively cooperate with BMP signaling. Primary cultures of lens cells were used to reveal an unprecedented type of cross-talk between the canonical FGF and BMP signaling pathways that regulates lens cell differentiation and intercellular coupling.

Fibroblast growth factors (FGFs) play a central role in two processes essential for lens transparency—fiber cell differentiation and gap junction–mediated intercellular communication (GJIC). Using serum-free primary cultures of chick lens epithelial cells (DCDMLs), we investigated how the FGF and bone morphogenetic protein (BMP) signaling pathways positively cooperate to regulate lens development and function. We found that culturing DCDMLs for 6 d with the BMP blocker noggin inhibits the canonical FGF-to-ERK pathway upstream of FRS2 activation and also prevents FGF from stimulating FRS2- and ERK-independent gene expression, indicating that BMP signaling is required at the level of FGF receptors. Other experiments revealed a second type of BMP/FGF interaction by which FGF promotes expression of BMP target genes as well as of BMP4. Together these studies reveal a novel mode of cooperation between the FGF and BMP pathways in which BMP keeps lens cells in an optimally FGF-responsive state and, reciprocally, FGF enhances BMP-mediated gene expression. This interaction provides a mechanistic explanation for why disruption of either FGF or BMP signaling in the lens leads to defects in lens development and function.

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

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

A gradient of matrix-bound FGF-2 and perlecan is available to lens epithelial cells

Fibroblast growth factors play a key role in regulating lens epithelial cell proliferation and differentiation via an anteroposterior gradient that exists between the aqueous and vitreous humours. FGF-2 is the most important for lens epithelial cell proliferation and differentiation. It has been proposed that the presentation of FGF-2 to the lens epithelial cells involves the lens capsule as a source of matrix-bound FGF-2. Here we used immunogold labelling to measure the matrix-bound FGF-2 gradient on the inner surface of the lens capsule in flat-mounted preparations to visualize the FGF-2 available to lens epithelial cells. We also correlated FGF-2 levels with levels of its matrix-binding partner perlecan, a heparan sulphate proteoglycan (HSPG) and found the levels of both to be highest at the lens equator. These also coincided with increased levels of phosphorylated extracellular signal-regulated kinase 1 and 2 (pERK1/2) in lens epithelial cells that localised to condensed chromosomes of epithelial cells that were Ki-67 positive. The gradient of matrix-bound FGF-2 (anterior pole: 3.7 ± 1.3 particles/μm²; equator: 8.2 ± 1.9 particles/μm²; posterior pole: 4 ± 0.9 particles/μm²) and perlecan (anterior pole: 2.1 ± 0.4 particles/μm²; equator: 5 ± 2 particles/μm²; posterior pole: 1.9 ± 0.7 particles/μm²) available at the inner lens capsule surface was measured for the bovine lens. These data support the anteroposterior gradient hypothesis and provide the first measurement of the gradient for an important morphogen and its HSPG partner, perlecan, at the epithelial cell-lens capsule interface.

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.

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.

New insights into the mechanism of lens development using zebra fish

On the basis of recent advances in molecular biology, genetics, and live-embryo imaging, direct comparisons between zebra fish and human lens development are being made. The zebra fish has numerous experimental advantages for investigation of fundamental biomedical problems that are often best studied in the lens. The physical characteristics of visible light can account for the highly coordinated cell differentiation during formation of a beautifully transparent, refractile, symmetric optical element, the biological lens. The accessibility of the zebra fish lens for direct investigation during rapid development will result in new knowledge about basic functional mechanisms of epithelia-mesenchymal transitions, cell fate, cell-matrix interactions, cytoskeletal interactions, cytoplasmic crowding, membrane transport, cell adhesion, cell signaling, and metabolic specialization. The lens is well known as a model for characterization of cell and molecular aging. We review the recent advances in understanding vertebrate lens development conducted with zebra fish.

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

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

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.

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.

Proliferative capacity of corneal endothelial cells

The corneal endothelial monolayer helps maintain corneal transparency through its barrier and ionic "pump" functions. This transparency function can become compromised, resulting in a critical loss in endothelial cell density (ECD), corneal edema, bullous keratopathy, and loss of visual acuity. Although penetrating keratoplasty and various forms of endothelial keratoplasty are capable of restoring corneal clarity, they can also have complications requiring re-grafting or other treatments. With the increasing worldwide shortage of donor corneas to be used for keratoplasty, there is a greater need to find new therapies to restore corneal clarity that is lost due to endothelial dysfunction. As a result, researchers have been exploring alternative approaches that could result in the in vivo induction of transient corneal endothelial cell division or the in vitro expansion of healthy endothelial cells for corneal bioengineering as treatments to increase ECD and restore visual acuity. This review presents current information regarding the ability of human corneal endothelial cells (HCEC) to divide as a basis for the development of new therapies. Information will be presented on the positive and negative regulation of the cell cycle as background for the studies to be discussed. Results of studies exploring the proliferative capacity of HCEC will be presented and specific conditions that affect the ability of HCEC to divide will be discussed. Methods that have been tested to induce transient proliferation of HCEC will also be presented. This review will discuss the effect of donor age and endothelial topography on relative proliferative capacity of HCEC, as well as explore the role of nuclear oxidative DNA damage in decreasing the relative proliferative capacity of HCEC. Finally, potential new research directions will be discussed that could take advantage of and/or improve the proliferative capacity of these physiologically important cells in order to develop new treatments to restore corneal clarity.

Homeostasis in the vertebrate lens: mechanisms of solute exchange

The eye lens is avascular, deriving nutrients from the aqueous and vitreous humours. It is, however, unclear which mechanisms mediate the transfer of solutes between these humours and the lens' fibre cells (FCs). In this review, we integrate the published data with the previously unpublished ultrastructural, dye loading and magnetic resonance imaging results. The picture emerging is that solute transfer between the humours and the fibre mass is determined by four processes: (i) paracellular transport of ions, water and small molecules along the intercellular spaces between epithelial and FCs, driven by Na(+)-leak conductance; (ii) membrane transport of such solutes from the intercellular spaces into the fibre cytoplasm by specific carriers and transporters; (iii) gap-junctional coupling mediating solute flux between superficial and deeper fibres, Na(+)/K(+)-ATPase-driven efflux of waste products in the equator, and electrical coupling of fibres; and (iv) transcellular transfer via caveoli and coated vesicles for the uptake of macromolecules and cholesterol. There is evidence that the Na(+)-driven influx of solutes occurs via paracellular and membrane transport and the Na(+)/K(+)-ATPase-driven efflux of waste products via gap junctions. This micro-circulation is likely restricted to the superficial cortex and nearly absent beyond the zone of organelle loss, forming a solute exchange barrier in the lens.

Planar cell polarity in the mammalian eye lens

The major role of the eye lens is to transmit and focus images onto the retina. For this function, the lens needs to develop and maintain the correct shape, notably, the precise curvature and high-level order and organization of its elements. The lens is mainly comprised of highly elongated fiber cells with hexagonal cross-sectional profiles that facilitate regular packing. Collectively, they form concentrically arranged layers around the anterior-posterior polar axis, and their convex curvature contributes to the spheroidal shape of the lens. Although the lens has been a popular system for developmental studies, little is known about the mechanism(s) that underlies the development of its exquisite three-dimensional cellular architecture. In this review, we will describe our recent work, which shows how planar cell polarity (PCP) operates in lens and contributes to its morphogenesis. We believe that the lens will be a useful model system to study PCP in general and gain insights into mechanisms that generate high-level cellular order during development.

Understanding the role of growth factors in embryonic development: insights from the lens

Growth factors play key roles in influencing cell fate and behaviour during development. The epithelial cells and fibre cells that arise from the lens vesicle during lens morphogenesis are bathed by aqueous and vitreous, respectively. Vitreous has been shown to generate a high level of fibroblast growth factor (FGF) signalling that is required for secondary lens fibre differentiation. However, studies also show that FGF signalling is not sufficient and roles have been identified for transforming growth factor-β and Wnt/Frizzled families in regulating aspects of fibre differentiation. In the case of the epithelium, key roles for Wnt/β-catenin and Notch signalling have been demonstrated in embryonic development, but it is not known if other factors are required for its formation and maintenance. This review provides an overview of current knowledge about growth factor regulation of differentiation and maintenance of lens cells. It also highlights areas that warrant future study.

A requirement for FGF signalling in the formation of primitive streak-like intermediates from primitive ectoderm in culture

BACKGROUND

Embryonic stem (ES) cells hold considerable promise as a source of cells with therapeutic potential, including cells that can be used for drug screening and in cell replacement therapies. Differentiation of ES cells into the somatic lineages is a regulated process; before the promise of these cells can be realised robust and rational methods for directing differentiation into normal, functional and safe cells need to be developed. Previous in vivo studies have implicated fibroblast growth factor (FGF) signalling in lineage specification from pluripotent cells. Although FGF signalling has been suggested as essential for specification of mesoderm and endoderm in vivo and in culture, the exact role of this pathway remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Using a culture model based on early primitive ectoderm-like (EPL) cells we have investigated the role of FGF signalling in the specification of mesoderm. We were unable to demonstrate any mesoderm inductive capability associated with FGF1, 4 or 8 signalling, even when the factors were present at high concentrations, nor any enhancement in mesoderm formation induced by exogenous BMP4. Furthermore, there was no evidence of alteration of mesoderm sub-type formed with addition of FGF1, 4 or 8. Inhibition of endogenous FGF signalling, however, prevented mesoderm and favoured neural differentiation, suggesting FGF signalling was required but not sufficient for the differentiation of primitive ectoderm into primitive streak-like intermediates. The maintenance of ES cell/early epiblast pluripotent marker expression was also observed in cultures when FGF signalling was inhibited.

CONCLUSIONS/SIGNIFICANCE

FGF signalling has been shown to be required for the differentiation of primitive ectoderm to neurectoderm. This, coupled with our observations, suggest FGF signalling is required for differentiation of the primitive ectoderm into the germ lineages at gastrulation.

Immunolocalization of FGF1 and FGF2 in the regenerating tail of the lizard Lampropholis guichenoti: implications for FGFs as trophic factors in lizard tail regeneration

A role for fibroblast growth factors in stimulating limb and tail regeneration in amphibians has been shown; however, it is unknown whether these growth factors are also involved in the regeneration of the tail of lizard, an amniote model for studies on tissue regeneration. The presence of fibroblast growth factor-1 (FGF1) and -2 (FGF2) in the regenerating tail of the lizard Lampropholis guichenoti has been studied using immunofluorescence labeling. The study reveals that FGF2 is mainly localized in the wound and scaling epidermis, in differentiating muscles, in spinal ganglia, regenerating nerves and spinal cord. FGF1 is also present in the wound and differentiating epidermis, but is detectable at lower levels in the regenerating muscles and spinal cord. FGF1 is present in blastema cells, while FGF2 labeling is relatively low in these cells. Fibroblasts of the forming dermis are rich in FGF1 but not in FGF2. Developing blood vessels label for both FGF1 and FGF2 while the cartilaginous, bone and fat tissues are poorly labeled or unlabeled for FGFs. The present study suggests that most FGFs in the regenerating tail are located in the nervous system, in the epidermis and muscles, and these tissues most likely require these growth factors for their differentiation and growth. The present study suggests that FGFs produced in the regenerating epidermis, spinal cord and nerves can stimulate tail regeneration in lizards.

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

The eye lens is an encapsulated avascular organ whose function is to focus light on the retina. Lens comprises a single progenitor cell lineage in multiple states of differentiation. Disruption of lens function leading to protein aggregation and opacity results in age-onset cataract. Cataract is a complex disease involving genetic and environmental factors. Here, we report the development of a new 3-stage system that differentiates human embryonic stem cells (hESCs) into large quantities of lens progenitor-like cells and differentiated 3-dimensional lentoid bodies. Inhibition of BMP signaling by noggin triggered differentiation of hESCs toward neuroectoderm. Subsequent reactivation of BMP and activation of FGF signaling stimulated formation of lens progenitor cells marked by the expression of PAX6 and alpha-crystallins. The formation of lentoid bodies was most efficient in the presence of FGF2 and Wnt-3a, yielding approximately 1000 lentoid bodies/30-mm well. Lentoid bodies expressed and accumulated lens-specific markers including alphaA-, alphaB-, beta-, and gamma-crystallins, filensin, CP49, and MIP/aquaporin 0. Collectively, these studies identify a novel procedure to generate lens cells from hESCs that can be applied for studies of lens differentiation and cataractogenesis using induced pluripotent stem (iPS) cells derived from various cataract patients.

Regulation of lens gap junctions by Transforming Growth Factor beta

Using cultured lens epithelial cells, we discovered a new type of cross-talk between the FGF and TGF-β pathways, as well as a novel role for TGF-β and p38 kinase in the regulation of gap junctional intercellular communication. Our findings provide an explanation for how pathologically increased TGF-β signaling could contribute to cataract formation.

Gap junction–mediated intercellular communication (GJIC) is essential for the proper function of many organs, including the lens. GJIC in lens epithelial cells is increased by FGF in a concentration-dependent process that has been linked to the intralenticular gradient of GJIC required for lens transparency. Unlike FGF, elevated levels of TGF-β are associated with lens dysfunction. We show that TGF–β1 or -2 up-regulates dye coupling in serum-free primary cultures of chick lens epithelial cells (dissociated cell-derived monolayer cultures [DCDMLs]) via a mechanism distinct from that utilized by other growth factors. Remarkably, the ability of TGF-β and of FGF to up-regulate GJIC is abolished if DCDMLs are simultaneously exposed to both factors despite undiminished cell–cell contact. This reduction in dye coupling is attributable to an inhibition of gap junction assembly. Connexin 45.6, 43, and 56–containing gap junctions are restored, and intercellular dye coupling is increased, if the activity of p38 kinase is blocked. Our data reveal a new type of cross-talk between the FGF and TGF-β pathways, as well as a novel role for TGF-β and p38 kinase in the regulation of GJIC. They also provide an explanation for how pathologically increased TGF-β signaling could contribute to cataract formation.

Secreted frizzled-related protein disrupts PCP in eye lens fiber cells that have polarised primary cilia

Planar cell polarity (PCP) signaling polarises cells along tissue axes. Although pathways involved are becoming better understood, outstanding issues include; (i) existence/identity of cues that orchestrate global polarisation in tissues, and (ii) the generality of the link between polarisation of primary cilia and asymmetric localisation of PCP proteins. Mammalian lenses are mainly comprised of epithelial-derived fiber cells. Concentrically arranged fibers are precisely aligned as they elongate along the anterior-posterior axis and orientate towards lens poles where they meet fibers from other segments to form characteristic sutures. We show that lens exhibits PCP, with each fiber cell having an apically situated cilium and in most cases this is polarised towards the anterior pole. Frizzled and other PCP proteins are also asymmetrically localised along the equatorial-anterior axis. Mutations in core PCP genes Van Gogh-like 2 and Celsr1 perturb oriented fiber alignment and suture formation. Suppression of the PCP pathway by overexpressing Sfrp2 shows that whilst local groups of fibers are often similarly oriented, they lack global orientation; consequently when local groups of fibers with different orientations meet they form multiple, small, ectopic suture-like configurations. This indicates that this extracellular inhibitor disrupts a global polarising signal that utilises a PCP-mediated mechanism to coordinate the global alignment and orientation of fibers to lens poles.

Notch signaling is required for lateral induction of Jagged1 during FGF-induced lens fiber differentiation

Previous studies of the developing lens have shown that Notch signaling regulates differentiation of lens fiber cells by maintaining a proliferating precursor pool in the anterior epithelium. However, whether Notch signaling is further required after the onset of fiber cell differentiation is not clear. This work investigates the role of Notch2 and Jagged1 (Jag1) in secondary fiber cell differentiation using rat lens epithelial explants undergoing FGF-2 dependent differentiation in vitro. FGF induced Jag1 expression and Notch2 signaling (as judged by the appearance of activated Notch2 Intracellular Domain (N2ICD)) within 12-24 h. These changes were correlated with induction of the Notch effector, Hes5, upregulation of N-cadherin (N-cad), and downregulation of E-cadherin (E-cad), a cadherin switch characteristic of fiber cell differentiation. Induction of Jag1 was efficiently blocked by U0126, a specific inhibitor of MAPK/ERK signaling, indicating a requirement for signaling through this pathway downstream of the FGF receptor. Other growth factors that activate MAPK/ERK signaling (EGF, PDGF, IGF) did not induce Jag1. Inhibition of Notch signaling using gamma secretase inhibitors DAPT and L-685,458 or anti-Jag1 antibody markedly decreased FGF-dependent expression of Jag1 demonstrating Notch-dependent lateral induction. In addition, inhibition of Notch signaling reduced expression of N-cad, and the cyclin dependent kinase inhibitor, p57Kip2, indicating a direct role for Notch signaling in secondary fiber cell differentiation. These results demonstrate that Notch-mediated lateral induction of Jag1 is an essential component of FGF-dependent lens fiber cell differentiation.

MAPK/ERK1/2 and PI3-kinase signalling pathways are required for vitreous-induced lens fibre cell differentiation

Lens epithelial cells withdraw from the cell cycle to differentiate into secondary fibre cells in response to vitreal factors. Fibroblast growth factor (FGF) in the vitreous has been shown to induce lens fibre differentiation in vivo and in vitro through the activation of defined intracellular signalling, namely via MAPK/ERK1/2 and PI3-K/Akt pathways. To better understand the role of these growth factor-activated signalling pathways in lens fibre differentiation, FGF- and vitreous-induced lens fibre differentiation was examined in primary rat lens epithelial cell explants. The induction of cell elongation and fibre specific beta- and gamma-crystallin expression in lens explants was accompanied by distinct phosphorylation profiles for ERK1/2 and Akt. Using selective inhibitors (U0126 and LY294002) in blocking studies, these pathways were shown to be required for different aspects of lens fibre differentiation. Furthermore, a short 'pulse' treatment of explants with FGF showed that the activation of ERK1/2 over 24 h was not sufficient for the progression of lens fibre differentiation and that cyclic ERK1/2 phosphorylation was required throughout the extended differentiation process. In conclusion, these results support a key role for both ERK1/2 and PI3-kinase/Akt signalling pathways in FGF- and vitreous-induced lens fibre differentiation.

Growth factors involved in aqueous humour-induced lens cell proliferation

Lens epithelial cell proliferation is regulated by growth factors in the aqueous humour of the eye. Although the lens fibre cell-differentiating factors are well defined, the factors in aqueous that promote lens cell proliferation are not. Mitogens present in aqueous primarily signal through the MAPK/ERK and PI3-K/Akt pathways. By characterising the signalling pathways involved in lens cell proliferation, we aim to identify the factors in aqueous that regulate this process in vivo. Using rat lens epithelial explants, 5'-2'-bromo-deoxyuridine and H(3)-thymidine incorporation were used to compare the effects of aqueous, insulin-like growth factor (IGF-1), platelet-derived growth factor (PDGF-A), epidermal growth factor (EGF) and fibroblast growth factor (FGF-2) on lens cell proliferation. Western blotting was employed to characterise ERK1/2 and Akt signalling induced by these mitogens. The above assays were also repeated in the presence of selective receptor inhibitors. Similar to aqueous, FGF induced a sustained ERK1/2 signalling profile (up to 6 h), unlike IGF, PDGF and EGF that induced a transient activation of ERK1/2. In the presence of a FGF receptor (FGFR) inhibitor, the sustained aqueous-induced ERK1/2 signalling profile was perturbed, resembling the transient IGF-, PDGF- or EGF-induced profile. In the presence of other growth factor receptor inhibitors, aqueous maintained its sustained, 6 h, ERK1/2 signalling profile, although ERK1/2 phosphorylation at earlier time periods was reduced. No one-specific receptor inhibitor could block aqueous-induced lens cell proliferation; however, combinations of inhibitors could, providing FGFR signalling was blocked. Multiple growth factors are likely to regulate lens cell proliferation in vivo, with a key role for FGF in aqueous-induced signalling and lens cell proliferation.

Essential role of BMPs in FGF-induced secondary lens fiber differentiation

It is widely accepted that vitreous humor-derived FGFs are required for the differentiation of anterior lens epithelial cells into crystallin-rich fibers. We show that BMP2, 4, and 7 can induce the expression of markers of fiber differentiation in primary lens cell cultures to an extent equivalent to FGF or medium conditioned by intact vitreous bodies (VBCM). Abolishing BMP2/4/7 signaling with noggin inhibited VBCM from upregulating fiber marker expression. Remarkably, noggin and anti-BMP antibodies also prevented purified FGF (but not unrelated stimuli) from upregulating the same fiber-specific proteins. This effect is attributable to inhibition of BMPs produced by the lens cells themselves. Although BMP signaling is required for FGF to enhance fiber differentiation, the converse is not true. Expression of noggin in the lenses of transgenic mice resulted in a postnatal block of epithelial-to-secondary fiber differentiation, with extension of the epithelial monolayer to the posterior pole of the organ. These results reveal the central importance of BMP in secondary fiber formation and show that although FGF may be necessary for this process, it is not sufficient. Differentiation of fiber cells, and thus proper vision, is dependent on cross-talk between the FGF and BMP signaling pathways.

Wnt signaling is required for organization of the lens fiber cell cytoskeleton and development of lens three-dimensional architecture

How an organ develops its characteristic shape is a major issue. This is particularly critical for the eye lens as its function depends on having appropriately ordered three-dimensional cellular architecture. Recent in vitro studies indicate that Wnt signaling plays key roles in regulating morphological events in FGF-induced fiber cell differentiation in the mammalian lens. To further investigate this the Wnt signaling antagonist, secreted frizzled-related protein 2 (Sfrp2), was overexpressed in lens fiber cells of transgenic mice. In these mice fiber cell elongation was attenuated and individual fibers exhibited irregular shapes and consequently did not align or pack regularly; microtubules, microfilaments and intermediate filaments were clearly disordered in these fibers. Furthermore, a striking feature of transgenic lenses was that fibers did not develop the convex curvature typically seen in normal lenses. This appears to be related to a lack of protrusive processes that are required for directed migratory activity at their apical and basal tips as well as for the formation of interlocking processes along their lateral margins. Components of the Wnt/Planar Cell Polarity (PCP) pathway were downregulated or inhibited. Taken together this supports a role for Wnt/PCP signaling in orchestrating the complex organization and dynamics of the fiber cell cytoskeleton.

Cross-talk between fibroblast growth factor and bone morphogenetic proteins regulates gap junction-mediated intercellular communication in lens cells

Homeostasis in the lens is dependent on an extensive network of cell-to-cell gap junctional channels. Gap junction-mediated intercellular coupling (GJIC) is higher in the equatorial region of the lens than at either pole, an asymmetry believed essential for lens transparency. Primary cultures of embryonic chick lens epithelial cells up-regulate GJIC in response to purified fibroblast growth factor (FGF)1/2 or to medium conditioned by vitreous bodies, the major reservoir of factors (including FGF) for the lens equator. We show that purified bone morphogenetic protein (BMP)2, -4, and -7 also up-regulate GJIC in these cultures. BMP2, -4, or both are present in vitreous body conditioned medium, and BMP4 and -7 are endogenously expressed by lens cells. Remarkably, lens-derived BMP signaling is required for up-regulation of GJIC by purified FGF, and sufficient for up-regulation by vitreous humor. This is the first demonstration of an obligatory interaction between FGF and BMPs in postplacode lens cells, and of a role for FGF/BMP cross-talk in regulating GJIC in any cell type. Our results support a model in which the angular gradient in GJIC in the lens, and thus proper lens function, is dependent on signaling between the FGF and BMP pathways.

Generation of transparency and cellular organization in lens explants

The lens grows via the proliferation and differentiation of lens epithelial cells into lens fibres. This differentiation process, thought to be controlled by factors present in the vitreous fluid, generates tightly-packed, parallel-aligned fibre cells that confer transparency to the lens. Using lens epithelial-cell explants we examined how explant orientation and growth factor treatment can affect cellular arrangement and explant transparency. Fibre cell differentiation was induced in lens explants by culturing cells with fibroblast growth factor (FGF) or bovine vitreous. Cell shape and arrangement was investigated using confocal microscopy, electron microscopy, immunofluorescence and in situ hybridization. Explant transparency was measured using light microscopy. Confocal microscopy demonstrated that explant orientation determined cellular arrangement, irrespective of the differentiation stimuli used. In explants where epithelial cells were confined between their normal basement membrane (the lens capsule) and the base of the culture dish, the cells became elongated, thin and parallel-aligned. In contrast, in explants cultured with cells directly exposed to the culture media the cells appeared to be shorter, globular and haphazardly arranged. FGF initiated the differentiation of most lens epithelial cells; however, abnormal cellular morphologies developed with subsequent culture of the cells. As a result, the transparency of these explants decreased with prolonged culture. Interestingly, explants cultured with vitreous (i) did not develop abnormal cellular morphologies, (ii) contained two distinct cell types (retained epithelial cells and newly differentiated fibre cells) and (iii) remained transparent throughout the lengthy culture period. In summary, we have developed a culture system that generates a transparent tissue with a cellular arrangement resembling that of the lens in vivo. We have shown that while FGF and vitreous initiate differentiation within this system, better maintenance of fibre cell integrity, more appropriate regulation of molecular events, and better maintenance of explant transparency was achieved in the presence of vitreous. This system offers an opportunity to further investigate the process of lens fibre cell differentiation as well as a means of better identifying the factors that contribute to the development of tissue transparency in vitro.

Fgf19 is required for zebrafish lens and retina development

Fgf signaling plays crucial roles in morphogenesis. Fgf19 is required for zebrafish forebrain development. Here, we examined the roles of Fgf19 in the formation of the lens and retina in zebrafish. Knockdown of Fgf19 caused a size reduction of the lens and the retina, failure of closure of the choroids fissure, and a progressive expansion of the retinal tissue to the midline of the forebrain. Fgf19 expressed in the nasal retina and lens was involved in cell survival but not cell proliferation during embryonic lens and retina development. Fgf19 was essential for the differentiation of lens fiber cells in the lens but not for the neuronal differentiation and lamination in the retina. Loss of nasal fate in the retina caused by the knockdown of Fgf19, expansion of nasal fate in the retina caused by the overexpression of Fgf19 and eye transplantation indicated that Fgf19 in the retina was crucial for the nasal-temporal patterning of the retina that is critical for the guidance of retinal ganglion cell axons. Knockdown of Fgf19 also caused incorrect axon pathfinding. The present findings indicate that Fgf19 positively regulates the patterning and growth of the retina, and the differentiation and growth of the lens in zebrafish.

The Etiology of Steroid Cataract

Steroid-induced posterior subcapsular cataracts (PSCs) exhibit three main distinctive characteristics: (i) association only with steroids possessing glucocorticoid activity, (ii) involvement of aberrant migrating lens epithelial cells, and (iii) a central posterior location. The first characteristic suggests a key role for glucocorticoid receptor activation and subsequent changes to the transcription of specific genes. Glucocorticoid receptor activation is associated in many cell types with proliferation, suppressed differentiation, a reduced susceptibility to apoptosis, altered transmembrane transport, and enhancement of reactive oxygen species activity. Glucocorticoids may be capable of inducing changes to the transcription of genes in lens epithelial cells that are related to many of these cellular processes. This review examines the various mechanisms that have been proposed to account for the development of PSC in the context of recent DNA array studies. Additionally, given that the glucocorticoid receptor can also engender wide-ranging indirect activities, glucocorticoids could also indirectly affect the lens through the responses of other cells within the ocular compartment and/or through effects on cells at more remote locations. These indirect mechanisms, which, for example, could be mediated through alterations to the intraocular levels of growth factors that normally orchestrate lens development and maintain lens homeostasis, are also discussed. Although the mechanism of steroid cataract induction remains unknown, glucocorticoid-induced gene transcription events in lens epithelial cells, and also other intraocular or systemic cells, likely interact to generate steroid cataracts. Finally, although evidence for glucocorticoid-protein adduct formation in the lens is inconclusive, the generation of such adducts cannot yet be discounted as a contributing factor and must necessarily be retained in discussions of the etiology of steroid cataract.