Fibre differentiation and polarity in the mammalian lens: a key role for FGF

Spatiotemporal Localisation of Heparan Sulphate Proteoglycans throughout Mouse Lens Morphogenesis

Heparan sulphate proteoglycans (HSPGs) consist of a core protein decorated with sulphated HS-glycosaminoglycan (GAG) chains. These negatively charged HS-GAG chains rely on the activity of PAPSS synthesising enzymes for their sulfation, which allows them to bind to and regulate the activity of many positively charged HS-binding proteins. HSPGs are found on the surfaces of cells and in the pericellular matrix, where they interact with various components of the cell microenvironment, including growth factors. By binding to and regulating ocular morphogens and growth factors, HSPGs are positioned to orchestrate growth factor-mediated signalling events that are essential for lens epithelial cell proliferation, migration, and lens fibre differentiation. Previous studies have shown that HS sulfation is essential for lens development. Moreover, each of the full-time HSPGs, differentiated by thirteen different core proteins, are differentially localised in a cell-type specific manner with regional differences in the postnatal rat lens. Here, the same thirteen HSPG-associated GAGs and core proteins as well as PAPSS2, are shown to be differentially regulated throughout murine lens development in a spatiotemporal manner. These findings suggest that HS-GAG sulfation is essential for growth factor-induced cellular processes during embryogenesis, and the unique and divergent localisation of different lens HSPG core proteins indicates that different HSPGs likely play specialized roles during lens induction and morphogenesis.

FGF-2 Differentially Regulates Lens Epithelial Cell Behaviour during TGF-β-Induced EMT

Fibroblast growth factor (FGF) and transforming growth factor-beta (TGF-β) can regulate and/or dysregulate lens epithelial cell (LEC) behaviour, including proliferation, fibre differentiation, and epithelial–mesenchymal transition (EMT). Earlier studies have investigated the crosstalk between FGF and TGF-β in dictating lens cell fate, that appears to be dose dependent. Here, we tested the hypothesis that a fibre-differentiating dose of FGF differentially regulates the behaviour of lens epithelial cells undergoing TGF-β-induced EMT. Postnatal 21-day-old rat lens epithelial explants were treated with a fibre-differentiating dose of FGF-2 (200 ng/mL) and/or TGF-β2 (50 pg/mL) over a 7-day culture period. We compared central LECs (CLECs) and peripheral LECs (PLECs) using immunolabelling for changes in markers for EMT (α-SMA), lens fibre differentiation (β-crystallin), epithelial cell adhesion (β-catenin), and the cytoskeleton (alpha-tropomyosin), as well as Smad2/3- and MAPK/ERK1/2-signalling. Lens epithelial explants cotreated with FGF-2 and TGF-β2 exhibited a differential response, with CLECs undergoing EMT while PLECs favoured more of a lens fibre differentiation response, compared to the TGF-β-only-treated explants where all cells in the explants underwent EMT. The CLECs cotreated with FGF and TGF-β immunolabelled for α-SMA, with minimal β-crystallin, whereas the PLECs demonstrated strong β-crystallin reactivity and little α-SMA. Interestingly, compared to the TGF-β-only-treated explants, α-SMA was significantly decreased in the CLECs cotreated with FGF/TGF-β. Smad-dependent and independent signalling was increased in the FGF-2/TGF-β2 co-treated CLECs, that had a heightened number of cells with nuclear localisation of Smad2/3 compared to the PLECs, that in contrast had more pronounced ERK1/2-signalling over Smad2/3 activation. The current study has confirmed that FGF-2 is influential in differentially regulating the behaviour of LECs during TGF-β-induced EMT, leading to a heterogenous cell population, typical of that observed in the development of post-surgical, posterior capsular opacification (PCO). This highlights the cooperative relationship between FGF and TGF-β leading to lens pathology, providing a different perspective when considering preventative measures for controlling PCO.

Radiation-Induced Alterations in Proliferation, Migration, and Adhesion in Lens Epithelial Cells and Implications for Cataract Development

The lens of the eye is one of the most radiosensitive tissues. Although the exact mechanism of radiation-induced cataract development remains unknown, altered proliferation, migration, and adhesion have been proposed as factors. Lens epithelial cells were exposed to X-rays (0.1-2 Gy) and radiation effects were examined after 12 h and 7 day. Proliferation was quantified using an MTT assay, migration was measured using a Boyden chamber and wound-healing assay, and adhesion was assessed on three extracellular matrices. Transcriptional changes were also examined using RT-qPCR for a panel of genes related to these processes. In general, a nonlinear radiation response was observed, with the greatest effects occurring at a dose of 0.25 Gy. At this dose, a reduction in proliferation occurred 12 h post irradiation (82.06 ± 2.66%), followed by an increase at 7 day (116.16 ± 3.64%). Cell migration was increased at 0.25 Gy, with rates 121.66 ± 6.49% and 232.78 ± 22.22% greater than controls at 12 h and 7 day respectively. Cell adhesion was consistently reduced above doses of 0.25 Gy. Transcriptional alterations were identified at these same doses in multiple genes related to proliferation, migration, and adhesion. Overall, this research began to elucidate the functional changes that occur in lens cells following radiation exposure, thereby providing a better mechanistic understanding of radiation-induced cataract development.

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.

The negative regulatory Spred1 and Spred2 proteins are required for lens and eye morphogenesis

The transparent and refractive properties of the ocular lens are dependent on its precise cellular structure, supported by the regulation of lens cellular processes of proliferation and differentiation that are essential throughout life. The ERK/MAPK-signalling pathway plays a crucial role in regulating lens cell proliferation and differentiation, and in turn is regulated by inhibitory molecules including the Spred family of proteins to modulate and attenuate the impact of growth factor stimulation. Given Spreds are strongly and distinctly expressed in lens, along with their established inhibitory role in a range of different tissues, we investigated the role these antagonists play in regulating lens cell proliferation and differentiation, and their contribution to lens structure and growth. Using established mice lines deficient for either or both Spred 1 and Spred 2, we demonstrate their role in regulating lens development by negatively regulating ERK1/2 activity. Mice deficient for both Spred 1 and Spred 2 have impaired lens and eye development, displaying irregular lens epithelial and fibre cell activity as a result of increased levels of phosphorylated ERK1/2. While Spred 1 and Spred 2 do not appear to be necessary for induction and early stages of lens morphogenesis (prior to E11.5), nor for the formation of the primary fibre cells, they are required for the continuous embryonic growth and differentiation of the lens.

Negative regulation of lens fiber cell differentiation by RTK antagonists Spry and Spred

Sprouty (Spry) and Spred proteins have been identified as closely related negative regulators of the receptor tyrosine kinase (RTK)-mediated MAPK pathway, inhibiting cellular proliferation, migration and differentiation in many systems. As the different members of this antagonist family are strongly expressed in the lens epithelium in overlapping patterns, in this study we used lens epithelial explants to examine the impact of these different antagonists on the morphologic and molecular changes associated with fibroblast growth factor (FGF)-induced lens fiber differentiation. Cells in lens epithelial explants were transfected using different approaches to overexpress the different Spry (Spry1, Spry2) and Spred (Spred1, Spred2, Spred3) members, and we compared their ability to undergo FGF-induced fiber differentiation. In cells overexpressing any of the antagonists, the propensity for FGF-induced cell elongation was significantly reduced, indicative of a block to lens fiber differentiation. Of these antagonists, Spry1 and Spred2 appeared to be the most potent among their respective family members, demonstrating the greatest block in FGF-induced fiber differentiation based on the percentage of cells that failed to elongate. Consistent with the reported activity of Spry and Spred, we show that overexpression of Spry2 was able to suppress FGF-induced ERK1/2 phosphorylation in lens cells, as well as the ERK1/2-dependent fiber-specific marker Prox1, but not the accumulation of β-crystallins. Taken together, Spry and Spred proteins that are predominantly expressed in the lens epithelium in situ, appear to have overlapping effects on negatively regulating ERK1/2-signaling associated with FGF-induced lens epithelial cell elongation leading to fiber differentiation. This highlights the important regulatory role for these RTK antagonists in establishing and maintaining the distinct architecture and polarity of the lens.

β1-Integrin Signaling is Essential for Lens Fiber Survival

Integrins have been proposed to play a major role in lens morphogenesis. To determine the role of β1-integrin and its down-stream signaling partner, integrin linked kinase (ILK), in lens morphogenesis, eyes of WT mice and mice with a nestin-linked conditional knockout of β1-integrin or ILK were analyzed for defects in lens development. Mice, lacking the genes encoding the p1-integrin subunit ( Itgb1) or ILK ( Ilk), showed a perinatal degeneration of the lens. Early signs of lens degeneration included vacuolization, random distribution of lens cell nuclei, disrupted fiber morphology and attenuation and separation of the lens capsule. The phenotype became progressively more severe during the first postnatal week eventually leading to the complete loss of the lens. A more severe phenotype was observed in ILK mutants at similar stages. Eyes from embryonic day 13 β1-integrin-mutant embryos showed no obvious signs of lens degeneration, indicating that mutant lens develops normally until peri-recombination. Our findings suggest that β1-integrins and ILK cooperate to control lens cell survival and link lens fibers to the surrounding extracellular matrix. The assembly and integrity of the lens capsule also appears to be reliant on integrin signaling within lens fibers. Extrapolation of these results indicates a novel role of integrins in lens cell-cell adhesions as well as a potential role in the pathogenesis of congenital cataracts.

Control of lens development by Lhx2-regulated neuroretinal FGFs

Fibroblast growth factor (FGF) signaling is an essential regulator of lens epithelial cell proliferation and survival, as well as lens fiber cell differentiation. However, the identities of these FGF factors, their source tissue and the genes that regulate their synthesis are unknown. We have found that Chx10-Cre;Lhx2lox/lox mice, which selectively lack Lhx2 expression in neuroretina from E10.5, showed an early arrest in lens fiber development along with severe microphthalmia. These mutant animals showed reduced expression of multiple neuroretina-expressed FGFs and canonical FGF-regulated genes in neuroretina. When FGF expression was genetically restored in Lhx2-deficient neuroretina of Chx10-Cre;Lhx2lox/lox mice, we observed a partial but nonetheless substantial rescue of the defects in lens cell proliferation, survival and fiber differentiation. These data demonstrate that neuroretinal expression of Lhx2 and neuroretina-derived FGF factors are crucial for lens fiber development in vivo.

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.

Dexamethasone influences FGF-induced responses in lens epithelial explants and promotes the posterior capsule coverage that is a feature of glucocorticoid-induced cataract

Aberrant spreading of lens epithelial cells along the posterior capsule is the basis for development of glucocorticoid (GC)-induced cataract; the resulting foci of nucleated cells at the posterior pole causing disruptions to normal lens cellular architecture. In this study, rat lens epithelial explants were used to assess the effects of dexamethasone (DEX), a widely used synthetic GC, on FGF2-induced lens cell proliferation and elongation as well as the ability of lens cells to spread and cover the posterior capsule. In the presence of FGF2, DEX significantly promoted lens cell proliferation after 48 h. Cell coverage of the posterior capsule was also enhanced during 5 days culture. In contrast, cell elongation was retarded by the inclusion of DEX. In the absence of FGF2, DEX had no marked effects on any of these cellular processes. Thus, in the presence of FGF2, DEX promoted cell proliferation and posterior capsule coverage but inhibited cell elongation. These results provide insights into the molecular mechanism underlying GC-induced cataract in humans.

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.

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.

Collagen and glycopolymer based hydrogel for potential corneal application

6-Methacryloyl-alpha-D-galactopyranose (MG) was synthesized, and characterized by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectrometry, and single-crystal X-ray diffraction. A series of interpenetrating polymer network (IPN) hydrogels was fabricated by simultaneously photocuring MG crosslinked by poly(ethylene glycol) diacrylate and chemically crosslinking type I collagen with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide. The successful incorporation of the glycopolymer, polymer MG, into collagen hydrogel was confirmed by FTIR and solid-state (13)C NMR. The optical characteristics of the IPN hydrogels are comparable to those of human corneas. The tensile strength and modulus of the hydrogels are enhanced by incorporation of polymer MG in comparison to that of the control collagen hydrogel. Biodegradation results indicated that polymer MG enhanced the stability of the composite hydrogels against collagenase. In vitro results demonstrated that the IPN hydrogel supported the adhesion and proliferation of human corneal epithelial cells and outperformed human cornea in blocking bacteria adhesion. Taken together, the IPN hydrogel might be a promising material for use in corneal lamellar keratoplasty.

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.

Differing effects of dexamethasone and diclofenac on posterior capsule opacification-like changes in a rat lens explant model

Posterior capsular opacification (PCO) arises from lens cells that remain associated with the lens capsule after cataract surgery and subsequently become abnormal, proliferate and migrate into the visual pathway. In this study, a rat lens explant model was used to assess the effects of the prototype steroidal and non-steroidal anti-inflammatory drugs, dexamethasone (DEX) and diclofenac (DIC), on epithelial cells undergoing PCO-like changes. Such drugs are widely used at the time of cataract surgery. TGFbeta2 and FGF-2 were added sequentially and explants were cultured for up to 30 days, with or without addition of DEX or DIC at a clinically relevant concentration. Without DEX or DIC, explants became multilayered and cells tended to retract into PCO-like plaques. Inclusion of DEX, but not DIC, resulted in transient formation of needle-like cells, enhanced cell coverage, and the retention a monolayer of migratory cells surrounding PCO-like plaques. With or without drug addition, most cells became aberrant, as indicated by loss of Pax6 expression and the presence of PCO markers alpha-smooth muscle actin and type I collagen; however, DEX and DIC both strongly enhanced type I collagen accumulation. Furthermore, DEX enhanced cell coverage in explants treated with TGFbeta alone. Thus the behaviour of lens cells was significantly and differentially affected by the presence of DEX and DIC, highlighting the possibility that drugs used to control inflammation after cataract surgery, and the clinician's choice of drugs, may influence PCO development.

Posterior capsule opacification-like changes in rat lens explants cultured with TGFbeta and FGF: effects of cell coverage and regional differences

Following cataract surgery, many patients suffer secondary loss of vision because of posterior capsule opacification (PCO), which arises when residual lens epithelial cells become aberrant and migrate into the light path. Transforming growth factor-beta (TGFbeta)-induced transdifferentiation of lens cells appears to play a key role in this process. Fibroblast growth factor (FGF) may also play a role by promoting the survival of TGFbeta-affected cells and influencing their subsequent behaviour. In the present study, the effects of two different TGFbeta and FGF treatment regimes were compared in rat lens epithelial explants with either low or high initial cell coverage. Explants treated with 50 pg ml(-1) TGFbeta2 and 20 ng ml(-1) FGF-2 sequentially (day 0, day 1) or simultaneously (day 0), then cultured for up to 30 days with FGF, were assessed by light microscopy and immunolocalisation of markers for transdifferentiation (alpha-smooth muscle actin (alphaSMA) and type I collagen) or lens epithelial phenotype (Pax6) and fibre differentiation (beta-crystallin). By day 4, most cells had lost Pax6 reactivity, alphaSMA reactivity was evident, and there were differences between growth factor treatment groups, low and high initial cell coverage explants, and peripheral and central regions of explants. On day 30 of culture, all explants were well populated with cells, irrespective of treatment and initial cell coverage, and exhibited diverse PCO-like morphological changes, with expression of transdifferentiation markers and beta-crystallin in virtually all cells. Such overall resilience to variations in conditions may contribute to the insidious nature of PCO, while factors related to observed early differences between groups may contribute to PCO pleiomorphism.

Laminin-binding integrins in rat lens morphogenesis and their regulation during fibre differentiation

Mammalian lens development involves cell-cell and cell-ECM interactions. As integrins are a major family of cell adhesion molecules, we examined the expression patterns of several integrin subunits (alpha3A, alpha3B, alpha6A, alpha6B, beta1 and beta4) during rat lens development. RT-PCR, in situ hybridisation, immunofluorescence and immunoblotting were used to investigate expression of integrin subunits during lens development and differentiation. RT-PCR showed expression of alpha3A, alpha6A, alpha6B and beta1A but not alpha3B or beta4 subunits in postnatal rat lenses. Each subunit displayed distinct spatio-temporal expression patterns. beta1 integrin was expressed in both epithelium and fibres. alpha3A subunit expression was restricted to the epithelium; expression ceased abruptly at the lens equator. Expression of the alpha6A subunit increased during fibre differentiation, whereas alpha6B expression was predominantly associated with epithelial cells during lens development. In lens epithelial explants, FGF induced some of the changes in integrin expression that are characteristic of fibre differentiation in vivo. One notable exception was the inability of FGF to reproduce the distinctive down-regulation of the alpha3 isoform that is associated with initiation of elongation in vivo. Interestingly, vitreous treatment was able to reproduce this shift in alpha3 expression indicating that another factor(s), in addition to FGF, may be required for full and complete transition from an epithelial cell to a fibre cell. Integrin subunit expression therefore appears to be highly regulated during lens development and fibre differentiation with evidence of major changes in alpha3 and alpha6 isoform expression. These results indicate that integrins may play important roles in development and growth of the lens. How specific integrin subunits influence the behaviour of cells in different developmental compartments of the lens remains to be determined.

Growth factor regulation of lens development

Lens arises from ectoderm situated next to the optic vesicles. By thickening and invaginating, the ectoderm forms the lens vesicle. Growth factors are key regulators of cell fate and behavior. Current evidence indicates that FGFs and BMPs are required to induce lens differentiation from ectoderm. In the lens vesicle, posterior cells elongate to form the primary fibers whereas anterior cells differentiate into epithelial cells. The divergent fates of these embryonic cells give the lens its distinctive polarity. There is now compelling evidence that, at least in mammals, FGF is required to initiate fiber differentiation and that progression of this complex process depends on the synchronized and integrated action of a number of distinct growth factor-induced signaling pathways. It is also proposed that an antero-posterior gradient of FGF stimulation in the mammalian eye ensures that the lens attains and maintains its polarity and growth patterns. Less is known about differentiation of the lens epithelium; however, recent studies point to a role for Wnt signaling. Multiple Wnts and their receptors are expressed in the lens epithelium, and mice with impaired Wnt signaling have a deficient epithelium. Recent studies also indicate that other families of molecules, that can modulate growth factor signaling, have a role in regulating the ordered growth and differentiation of the lens.

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

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

Expression and activation of the epidermal growth factor receptor in differentiating cells of the developing and post-hatching chicken lens

The epidermal growth factor receptor is hypothesized to play an important role in the post-natal growth and differentiation of the ocular lens. Immunohistochemistry and western blotting were utilized to examine the distribution and activation of the epidermal growth factor receptor in embryonic and post-hatching chicken lenses. Although present at constant levels within epithelial cells throughout embryonic development, the receptor becomes increasingly activated on a highly conserved tyrosine residue necessary for intracellular signal transduction as hatching approaches. After hatching, activated receptors are found in epithelial cells committed to fiber cell formation and in fiber cells undergoing initial stages of terminal differentiation. Activated receptors could not be identified in central epithelial cells or nuclear fiber cells. This pattern persists until at least one year post-hatching. These data indicate that the epidermal growth factor receptor is positioned to influence not only post-natal patterns of lenticular gene expression but also the greatest amount of lens growth and development.

Bi-directional migration of lens epithelial cells in a physiological electrical field

An endogenous electric field (EF) exists in the vertebrate lens, but the influences of this on the functions of lens epithelial cells (LECs) are unclear. Because LECs from different regions experience different patterns of endogenous EFs and show striking differences in morphology and function, bovine LECs from different regions were cultured in an applied EF to mimic that occurring naturally. The migration of LECs from central and peripheral regions was both stimulated and directed by an applied EF. Field-directed cell migration required serum, or growth factors. Cells cultured in serum free medium lost directed migration completely and this was restored partially by the addition of basic fibroblast growth factor. The direction of cell migration depended on both EF-strength and the origin of the LECs. Both central and peripheral LECs moved anodally at 150-250mVmm(-1). In a low EF (50mVmm(-1)), peripheral LECs migrated in the opposite direction, cathodally, but central LECs did not respond. Peripheral LECs alone therefore showed field strength dependent, bi-directional migration. A physiological EF enhanced activation of ERK1/2, a signaling molecule in the MAP kinase pathway, and this also required serum. Interestingly, expression of active ERK1/2 was enhanced in peripheral LECs at both 50 and 200mVmm(-1), which stimulated migration in different directions. In central LECs, which showed directed migration only at higher EFs, active ERK1/2 was increased only at 200mVmm(-1). The MAP kinase inhibitor U0126 prevented activation of ERK1/2 in LECs and inhibited EF-directed migration, implicating MAP kinase signaling in directing the migration of LECs in a physiological EF.

Changes in three types of ubiquitin mRNA and ubiquitin-protein conjugate levels during lens development

Ubiquitin is a small, highly conserved protein that covalently attaches to other proteins to form a unique branched protein structure. The best characterized function of this post-translational modification is to mark the modified protein for degradation by the proteasome. To investigate whether ubiquitin genes are regulated in lens development, the authors analyzed the levels of three ubiquitin mRNAs (UbA(52), UbB and UbC) in freshly dissected fiber and epithelial cells, and in epithelial explants induced to differentiate ex vivo. Explants, comprising the capsule and adherent epithelial cells, were dissected from lenses of 3 day old Sprague Dawley rats and cultured +/-bFGF to induce differentiation. Quantitative competitive RT-PCR was used to determine the mRNA levels in fresh and cultured cells. UbA(52), UbB and UbC mRNAs were 3.2 (P < 0.0001), 5.0 (P < 0.0001) and 6.8 (P < 0.0001) fold higher, respectively, in freshly dissected epithelial cells than in differentiated fiber cells. Immunological spot assays showed that ubiquitin protein is over two fold as high in rat pup lens epithelial cells as in fiber cells. The ubiquitin protein in fiber cells of adult rat is lower than that in adult epithelium and in pup fiber cells, indicating that ubiquitin content further decreased during lens fiber maturation. Western blots showed a greater amount of protein-conjugated ubiquitin (MW > 81 kD) in epithelial cells than in fiber cells, demonstrating a parallel pattern between the expression of ubiquitin mRNA, the level of ubiquitin protein and the level of conjugates in the cells. Epithelial cell explant cultures permit study of cells initiating differentiation. In contrast to fully differentiated fiber cells, explant cultures induced to initiate differentiation underwent differential up-regulation of ubiquitin gene expression. UbA(52) and UbB mRNA levels in +bFGF (differentiating) explant cultures were 2.6 (P < 0.001) and 1.4 (P < 0.001) fold higher, respectively, than those of -bFGF cultures. UbC mRNA content was similar in explants cultured with or without bFGF. Dissection of the isolated epithelial cells into regions representing distinct populations gave results consistent with this observation of the explant results. UbA(52), UbB and UbC mRNAs are 2.0, 2.2 and 1.76 fold higher, respectively, in the peripheral (initiating differentiation) than in the central (undifferentiated) region of epithelial cells. These results together indicate that UbA(52) and UbB mRNAs are transiently increased during the initiation and early stages of differentiation. However, UbC mRNA appears to be relatively unaffected at the earliest stage in this differentiation model and may have a different distribution than UbA(52) and UbB in the anterior lens cells. These data are consistent with an important role for ubiquitin during the early stages of lens differentiation. The selective expression indicates that the three genes have specific differentiation related functions.

Apoptosis is a feature of TGF beta-induced cataract

BACKGROUND

Studies in our laboratory have shown that transforming growth factor beta (TGF beta) induces rodent lens epithelial cells to undergo aberrant growth and differentiation that reproduces morphological and molecular features of human anterior subcapsular cataract and posterior capsule opacification. In addition, features of apoptosis have been described in some forms of human cataract. In the present study we investigated apoptotic changes induced by TGF beta in our rodent models.

METHODS

Lens epithelial explants and whole lenses from rats were cultured with TGF beta. Morphological analysis and TUNEL were used to detect apoptotic changes. Transgenic mice expressing a self-activating form of human TGF beta 1 in the lens were included in the analysis.

RESULTS

TGF beta-induced cell loss in epithelial explants coincided with increased numbers of pyknotic nuclei. Some of these nuclei were TUNEL-positive. Studies on lenses cultured with TGF beta and lenses from transgenic mice showed that the subcapsular plaques that developed contained pyknotic nuclei and that many of these were TUNEL-positive.

DISCUSSION

This study shows that cells develop morphological and molecular features of apoptosis in TGF beta-induced cataract models. This confirms that apoptosis can be included as another TGF beta-induced cellular change that mimics events in human cataract.

FGF-induced lens cell proliferation and differentiation is dependent on MAPK (ERK1/2) signalling

Members of the fibroblast growth factor (FGF) family induce lens epithelial cells to undergo cell division and differentiate into fibres; a low dose of FGF can stimulate cell proliferation (but not fibre differentiation), whereas higher doses of FGF are required to induce fibre differentiation. To determine if these cellular events are regulated by the same signalling pathways, we examined the role of mitogen-activated protein kinase (MAPK) signalling in FGF-induced lens cell proliferation and differentiation. We show that FGF induced a dose-dependent activation of extracellular regulated kinase 1/2 (ERK1/2) as early as 15 minutes in culture, with a high (differentiating) dose of FGF stimulating a greater level of ERK phosphorylation than a lower (proliferating) dose. Subsequent blocking experiments using UO126 (a specific inhibitor of ERK activation) showed that activation of ERK is required for FGF-induced lens cell proliferation and fibre differentiation. Interestingly, inhibition of ERK signalling can block the morphological changes associated with FGF-induced lens fibre differentiation; however, it cannot block the synthesis of some of the molecular differentiation markers, namely, β-crystallin. These findings are consistent with the in vivo distribution of the phosphorylated (active) forms of ERK1/2 in the lens. Taken together, our data indicate that different levels of ERK signalling may be important for the regulation of lens cell proliferation and early morphological events associated with fibre differentiation; however, multiple signalling pathways are likely to be required for the process of lens fibre differentiation and maturation.

Chondroitin sulphate proteoglycan is involved in lens vesicle morphogenesis in chick embryos

Proteoglycans have been implicated in the invagination and formation of various embryonal cavitied primordia. In this paper the expression of chondroitin sulphate proteoglycan (CSPG) is analysed in the lens primordium during lens vesicle formation, and demonstrate that this proteoglycan has a specific distribution pattern with regard to invagination and fusion processes in the transformation of placode into lens vesicle. More specifically, CSPG was detected in: (1) the apical surface of lens epithelial cells, where early CSPG expression was observed in the whole of the lens placode whilst in the vesicle phase it was restricted to the posterior epithelium; (2) intense CSPG expression in the basal lamina, which remained constant for the entire period under study; (3) CSPG expression in the intercellular spaces of the lens primordium epithelium, which increased during the invagination of the primordium and which at the vesicle stage was more evident in the posterior epithelium; and (4) CSPG expression on the edges of the lens placode both prior to and during fusion. Treatment with beta- D -xyloside causes significant CSPG depletion in the lens primordium together with severe alterations in the invagination and fusion of the lens vesicle; this leads to the formation of lens primordia which in some cases remain practically flat or show partial invagination defects or fusion disruption. Similar results were obtained by enzyme digestion with chondroitinase AC but not with type II heparinase, which indicates that alterations induced by beta- D -xyloside were due to interference in CSPG synthesis. The findings demonstrate that CSPG is a common component of the lens primordium at the earliest developmental stages during which it undergoes specific modifications. It also includes experimental evidence to show that 'in vivo' CSPG plays an important role in the invagination and fusion processes of the lens primordium.

Tgfbeta receptor expression in lens: implications for differentiation and cataractogenesis

TGFbeta induces changes characteristic of some forms of cataract. However, the responsiveness of lens epithelial cells to TGFbeta is age-dependent; weanling and adult, but not neonatal, lens epithelial cells respond. This study investigated TGFbeta receptor (TbetaRI and TbetaRII) expression during rat lens development and the effects of FGF-2 on TGFbeta responsiveness and TbetaR expression. Immunofluorescence, immunoblotting, RT-PCR and in situ hybridization were used to examine the spatio-temporal expression patterns of TbetaR. Lens explants were used to investigate the effects of FGF-2 on TGFbeta responsiveness and TbetaR expression. In the lens epithelium, little or no immunoreactivity was detected at P3 but at P21 there was distinct reactivity for TbetaRI and TbetaRII. Reactivity for both receptors was also found in the differentiating fibers in the transitional zone and cortex at both ages. Western blotting of lens membrane extracts identified multiple molecular weight forms of TbetaRI (30, 50, 90 kDa) and TbetaRII (70-120 kDa). In situ hybridization with a rat probe for Alk5 (TbetaRI) showed that the lens expresses Alk5 mRNA in epithelium and fibers throughout development. A rat TbetaRII probe revealed distinct expression of a TbetaRII mRNA in lens fibers throughout development and in the lens epithelium at P21 but not at P3. In vitro studies showed that lens epithelial explants from P9 rats did not undergo cataractous changes in response to TGFbeta but P13 explants did. Addition of FGF-2 to P9 explants induced increased TbetaR immunoreactivity and enhanced the competency of lens epithelial cells to respond to TGFbeta. These data indicate that the overall increased expression of TGFbeta receptors in lens epithelium during postnatal development (P3-P21) underlies an age-related change in TGFbeta responsiveness. The results also suggest that lens cells may express multiple forms of TbetaR. Expression of TbetaR in lens fibers throughout lens development and the induction of enhanced TbetaR expression by FGF suggest a role for TGFbeta signaling during FGF-induced responses and fiber differentiation.

Multiple functions of fibroblast growth factor-8 (FGF-8) in chick eye development

Fibroblast growth factor-8 (FGF-8) is an important signaling molecule in the generation and patterning of the midbrain, tooth, and limb. In this study we show that it is also involved in eye development. In the chick, Fgf-8 transcripts first appear in the distal optic vesicle when it contacts the head ectoderm. Subsequently Fgf-8 expression increases and becomes localized to the central area of the presumptive neural retina (NR) only. Application of FGF-8 has two main effects on the eye. First, it converts presumptive retinal pigment epithelium (RPE) into NR. This is apparent by the failure to express Bmp-7 and Mitf (a marker gene for the RPE) in the outer layer of the optic cup, coupled with the induction of NR genes, such as Rx, Sgx-1 and Fgf-8 itself. The induced retina displays the typical multilayered cytoarchitecture and expresses late neuronal differentiation markers such as synaptotagmin and islet-1. The second effect of FGF-8 exposure is the induction of both lens formation and lens fiber differentiation. This is apparent by the expression of a lens specific marker, L-Maf, and by morphological changes of lens cells. These results suggest that FGF-8 plays a role in the initiation and differentiation of neural retina and lens.

Insulin and IGF-I affect the protein composition of the lens fibre cell with possible consequences for cataract

Explanted newborn rat lens epithelial cells were cultured with various concentrations of FGF-2 and/or insulin or IGF-I for 8-20 days. The accumulation of alphaA-, alphaB-, betaA3/1-, betaB2- and gammaA-F-crystallin was measured. During culture with insulin only, i.e. in the absence of fibre cell differentiation, alphaA- and alphaB-crystallin accumulated to the same level as found in differentiating cells. Culture of epithelial cells with IGF-I led to an increase in alphaB-crystallin, but not in alphaA-crystallin. The addition of insulin under differentiation conditions (in the presence of 25 ng ml(-1)FGF-2) augmented the accumulation of alphaA-crystallin 1.5-fold, the accumulation of betaB2-crystallin two-fold and the accumulation of gammaA-F-crystallin five-fold over that found with FGF-2 only. The accumulation of alphaB- and betaA3/1-crystallin was not affected by insulin in the presence of FGF-2. Adding IGF-I to fibre cells differentiating in the presence of 25 ng ml(-1)FGF-2 resulted in a 1.5-fold increase (of questionable statistical significance) in both alphaA- and alphaB-crystallin and a two to three-fold increase in gammaA-F-crystallin compared to cells cultured with FGF-2 only, no significant effect of IGF-I on the accumulation of betaA3/1- or betaB2-crystallin was found. Comparison of the levels of mRNA and protein suggests that insulin acts to increase the level of transcription. Our results show that the response of fibre cells to insulin or IGF-I differs. Hence, even though half the maximum dosage required for the insulin effect was rather high (between 0.1 and >5 micro g), the effect of insulin cannot be merely transmitted by the IGF-I receptor. Our data further predict that insulin or IGF-I increases the overall ratio of beta- and gamma-crystallin to alpha-crystallin in the fibre cell, which could predispose the lens to cataract.

Disruption of lens fiber cell differentiation and survival at multiple stages by region-specific expression of truncated FGF receptors

To determine if fibroblast growth factor signaling mechanisms are required for terminal differentiation and survival of lens fiber cells, we evaluated the effects of expressing truncated fibroblast growth factor receptors (tFGFRs) in different regions of the developing lens. Two sets of transgenic mice were generated, one expressing tFGFRs from the alphaA-crystallin promoter (alphaA-tFGFR), which expresses linked genes in fiber cells throughout their differentiation program, and the other expressing tFGFRs from the gammaF-crystallin promoter (gammaF-tFGFR), which expresses linked genes beginning later during their differentiation. Histological and TUNEL analyses of lenses from alphaA-tFGFR and gammaF-tFGFR transgenic mice suggest that FGFR signaling is required for both early and late fiber cell differentiation and/or survival of the terminally differentiated cells. Additionally, multilayering and increased levels of apoptosis were observed in the anterior epithelium after the onset of fiber cell abnormalities. In situ hybridizations suggest that tFGFR transgenes were not expressed at significant levels in the epithelium. Combined with TUNEL and X-gal analyses on the lens epithelium from gammaF-tFGFR/Rosabeta-geo26 and nontransgenic/Rosabeta-geo26 chimeras, these results suggest that the organization and survival of the epithelial cells depend on appropriate structure and/or function of the differentiated fiber cells.

Lens development

This review gives a brief account of the main processes of lens development, including induction, morphogenesis, differentiation and growth. It describes what is known about the molecules and mechanisms that control and regulate these processes. Some of the recent progress made in understanding the molecular basis of lens development is highlighted along with some of the challenging areas for future research.

Induction and maintenance of differentiation of rat lens epithelium by FGF-2, insulin and IGF-1

The differentiation of rat lens epithelial cells to fibre cells can be mimicked using lens epithelial explants, which differentiate in vitro when exposed to fibroblast growth factor (FGF). A previous study demonstrated that FGF is required only for initiation of differentiation: once induced by FGF, differentiation can be maintained by insulin (as assessed by following the accumulation of fibre-cell specific crystallins). The aim of this investigation was to determine whether insulin-like growth factor 1 (IGF-1) can also maintain differentiation and to include a cellular analysis of explants undergoing insulin-or IGF-maintained differentiation in vitro. Measurement of the accumulation of alpha-, beta- and gamma-crystallins showed that IGF-1, like insulin, can replace FGF-2 in directing the pulses of alpha-, beta- and gamma-crystallin gene expression once differentiation is initiated by FGF-2. Cells in both the peripheral and the central region of the explants responded. Immunolocalization of alpha, beta- and gamma-crystallins in these explants showed that a 15 min pulse of FGF-2 triggered the differentiation of only a few cells, whereas a 12 hr pulse primed virtually all the cells for differentiation. This indicates that in explants, individual cells differ in the rate at which they can respond to FGF-2.

Regulation of expression within a gene family. The case of the rat gammaB- and gammaD-crystallin promoters

The six closely related and clustered rat gamma-crystallin genes, the gammaA- to gammaF-crystallin genes, are simultaneously activated in the embryonic lens but differentially shut down during postnatal development with the gammaB-crystallin gene, the last one to be active. We show here that developmental silencing of the gammaD-crystallin promoter correlates with delayed demethylation during lens fiber cell differentiation. Methylation silencing of the gammaD-crystallin promoter is a general effect and does not require the methylation of a specific CpG, nor does methylation interfere with factor binding to the proximal activator. In later development, the gammaD-crystallin promoter is also shut down earlier by a repressor that footprints to the -91/-78 region. A factor with identical properties is present in brain. Hence, a ubiquitous factor has been recruited as a developmental regulator by the lens. All gamma-crystallin promoters tested contain upstream silencers, but at least the gammaB-crystallin silencer is distinct from the gammaD-crystallin silencer. The gamma-crystallin promoters were found to share a proximal activator (the gamma-box; around -50), which behaves as a MARE. The gammaB-box is recognized with much lower avidity than the gammaD-box. By swapping elements between the gammaB- and the gammaD-crystallin promoter, we show that activation by the gammaB-box requires a directly adjacent -46/-38 AP-1 consensus site. These experiments also uncovered another positive element in the gammaD-crystallin promoter, around -10. In the context of the gammaD-crystallin promoter, this element is redundant; in the context of the gammaB-crystallin promoter, it can replace the -46/-38 element.

Synergism between temporally distinct growth factors: bFGF, insulin and lens cell differentiation

Fibroblast growth factors (FGFs) are the only known factors that can induce differentiation of the mammalian lens epithelial cell, while insulin acts only as a mitogen, not as a morphogen. We show here that insulin enhances expression of the alphaA-crystallin gene in lens epithelial cells and induces the synthesis of lens fibre cell specific betaB2- and gamma-crystallins in early differentiated fibre cells. Different signal transduction pathways are required for bFGF or insulin maintained fibre cell differentiation. A 15 min preincubation with bFGF was sufficient for the lens epithelial cells to become competent to undergo insulin maintained differentiation. The phorbol ester TPA could replace bFGF. The bFGF instructed competence to differentiate decays with a half-life of about 30 h. Hence, bFGF and insulin can act in concert to produce a differentiated phenotype even when they are not present simultaneously.