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

Duration of ERK1/2 phosphorylation induced by FGF or ocular media determines lens cell fate

The ocular environment is important for the establishment and maintenance of lens growth patterns and polarity. In the anterior chamber of the eye, the aqueous humour regulates lens epithelial cell proliferation whereas in the posterior, the vitreous humour regulates the differentiation of the lens cells into fiber cells. Members of the fibroblast growth factor (FGF) growth factor family have been shown to induce lens epithelial cells to undergo cell division and differentiate into fibers, with a low dose of FGF able to induce cell proliferation (but not fiber differentiation), and higher doses required to induce fiber differentiation. Both these cellular events have been shown to be regulated by the MAPK/ERK1/2 signalling pathway. In the present study, to better understand the contribution of ERK1/2 signalling in regulating lens cell proliferation and differentiation, we characterized the ERK1/2 signalling profiles induced by different doses of FGF, and compared these to those induced by the different ocular media. Here, we show that FGF induced a dose-dependent sustained activation of ERK1/2, with both a high (fiber differentiating) dose of FGF and vitreous, stimulating and maintaining a prolonged (up to 18 hr) ERK1/2 phosphorylation profile. In contrast, a lower (proliferating) dose of FGF, and aqueous, stimulated ERK1/2 phosphorylation for only up to 6 hr. If we selectively reduce the 18 hr ERK1/2 phosphorylation profile induced by vitreous to 6 hr, by specifically blocking FGF receptor signalling, the vitreous now fails to induce lens fiber differentiation but retains the ability to induce lens cell proliferation. These findings not only provide insights into the important role that FGF plays in the different ocular media that bathe the lens, but enlighten us on some of the putative molecular mechanisms by which one specific growth factor, in this case FGF, can elicit a different cellular response in the same cell type.

Cell cycle regulation in the developing lens

Regulation of cell proliferation is a critical aspect of the development of multicellular organisms. The ocular lens is an excellent model system in which to unravel the mechanisms controlling cell proliferation during development. In recent years, several cell cycle regulators have been shown to be essential for maintaining normal patterns of lens cell proliferation. Additionally, many growth factor signaling pathways and cell adhesion factors have been shown to have the capacity to regulate lens cell proliferation. Given this complexity, understanding the cross talk between these many signaling pathways and how they are coordinated are important directions for the future.

An essential role for FGF receptor signaling in lens development

Since the days of Hans Spemann, the ocular lens has served as one of the most important developmental systems for elucidating fundamental processes of induction and differentiation. More recently, studies in the lens have contributed significantly to our understanding of cell cycle regulation and apoptosis. Over 20 years of accumulated evidence using several different vertebrate species has suggested that fibroblast growth factors (FGFs) and/or fibroblast growth factor receptors (FGFRs) play a key role in lens development. FGFR signaling has been implicated in lens induction, lens cell proliferation and survival, lens fiber differentiation and lens regeneration. Here we will review and discuss historical and recent evidence suggesting that (FGFR) signaling plays a vital and universal role in multiple aspects of lens development.

Extracellular matrix and integrin signaling in lens development and cataract

During development of the vertebrate lens there are dynamic interactions between the extracellular matrix (ECM) of the lens capsule and lens cells. Disruption of the ECM causes perturbation of lens development and cataract. Similarly, changes in cell signaling can result in abnormal ECM and cataract. Integrins are key mediators of ECM signals and recent studies have documented distinct repertoires of integrin expression during lens development, and in anterior subcapsular cataract (ASC) and posterior caspsule opacification (PCO). Increasingly, studies are being directed to investigating the signaling pathways that integrins modulate and have identified Src, focal adhesion kinase (FAK) and integrin-linked kinase (ILK) as downstream kinases that mediate proliferation, differentiation and morphological changes in the lens during development and cataract formation.

Sef and Sprouty expression in the developing ocular lens: implications for regulating lens cell proliferation and differentiation

In many developmental systems, growth factor signalling must be temporally and spatially regulated, and this is commonly achieved by growth factor antagonists. Here, we describe the expression patterns of newly identified growth factor inhibitors, Sprouty and Sef, in the developing ocular lens. Sprouty and Sef are both expressed in the lens throughout embryogenesis, and become restricted to the lens epithelium, indicating that lens cell proliferation and fibre differentiation may be tightly regulated by such antagonists. Future studies will be aimed at understanding how these negative regulatory molecules modulate growth factor-induced signalling pathways and cellular processes in the lens.

Aqueous humour- and growth factor-induced lens cell proliferation is dependent on MAPK/ERK1/2 and Akt/PI3-K signalling

The aqueous humour of the eye is a rich source of growth factors, many of which have been shown to be lens cell mitogens; however, the identity of the endogenous mitogen(s) for lens cells is still unknown. As a first approach to identify the mechanisms by which these aqueous humour-derived growth factors induce lens cell proliferation, the present study set out to examine MAPK/ERK1/2 and PI3-K/Akt signalling associated with lens cell proliferation. Using a lens explant system, we examined the effects of different lens mitogens (aqueous humour, FGF, PDGF, IGF and EGF) using 5'-2'-bromo-deoxyuridine incorporation. In addition, we adopted immunolabelling techniques to compare the roles that the ERK1/2 and PI3-K signalling pathways play in regulating lens cell proliferation. We showed that the aqueous humour, and all the other growth factors examined, could activate ERK1/2 and PI3-K/Akt signalling. By targeting these pathways using specific pharmacological inhibitors, we were able to show that both ERK1/2 and PI3-K signalling are required for growth factor-induced lens cell proliferation, and that there was a strong correlation between the spatial distribution of proliferating cells in lens explants with ERK1/2 labelling. Furthermore, our blocking studies confirmed that PI3-K/Akt signalling can act upstream of ERK1/2, potentiating ERK1/2 phosphorylation in growth factor-induced lens cell proliferation. A better understanding of the signalling pathways required for aqueous humour-induced lens cell proliferation may ultimately allow us to identify the mitogen(s) that are important for regulating lens cell proliferation in situ.

Lens opacity and photoreceptor degeneration in the zebrafish lens opaque mutant

The zebrafish lens opaque (lop) mutant was identified in a chemical mutagenesis screen. The lop mutant, which develops normally through 4 days postfertilization (dpf), exhibits several signs of lens and retinal degeneration at 7 dpf. Histology revealed disrupted lens fibers and increased numbers of nucleated cells within the mutant lens and anterior chamber. The mutant lens also exhibited aberrant epithelial cell morphologies and lacked a definitive transition zone, which suggests that secondary fiber differentiation was interrupted. In addition, the mutant exhibits severely reduced photoreceptors and a reduction in the number of horizontal cells at 7 dpf. Other retinal cell classes appeared unaffected in the mutant. Transmission electron microscopy and opsin immunohistochemistry showed that the different photoreceptor types were generated at the retinal margin, but the rods and cones failed to mature and disappeared. The mutant lens and retina also displayed increased cell proliferation based on proliferating cell nuclear antigen immunolabeling, suggesting that the lens opacity was due to unregulated cell proliferation and undifferentiated cell accumulation within the mutant lens. The lop mutant phenotype supports recent studies showing the lens has a role in regulating teleost retinal development.

The involvement of neural retina pax6 in lens fiber differentiation

Proper eye formation depends on specific interactions between neural and ectodermal tissues coupled with temporally distinct gene expression and a regulated sequence of signaling events. The homeobox gene Pax6 is vitally important to the entire process of eye development in both vertebrates and invertebrates. Pax6 expression for the retina anlage has been shown to be indispensable in the development of various retinal cells. Here, we report that Pax6 expression in neural tissue plays an important role in lens development. Expression of a dominant-negative version of Pax6 isoform that lacks 5a-exon sequence in developing optic vesicles (OV) of chick embryos led to arrest of lens development at the lens vesicle stage as well as optic cup deformation. To gain insights into the molecular events underlying deformed lens formation, we examined the expression of several transcription factors in the lens of Pax6-negative-OV eye. Importantly, L-Maf was downregulated while c-Maf was found normal in deformed lens. We detected a downregulation of fibroblast growth factor (FGF8) in the neural tissue. Our in vivo experiments suggest that Pax6 in neural retina regulates FGF8 expression, which may maintain L-Maf expression in the lens to be essential for later lens fiber differentiation.

Lens epithelium supports axonal regeneration of retinal ganglion cells in a coculture model in vitro

The purpose of this study was to determine whether the lens epithelium influences the survival or axonal growth of regenerating retinal ganglion cells. The optic nerves of adult albino rats were injured in order to induce axonal regeneration, and axon growth was then studied in retinal explants in the presence of cocultivated lens capsules carrying living epithelial cells. In the first series of experiments, cocultivation of retinal explants with lens epithelium in immediate proximity resulted in penetration of fibers into the lens epithelium, indicating that it supported axonal growth. In the second series of experiments, co-explants were placed 0.5-1.0mm from each other. The numbers of outgrowing retinal axons were determined both with respect to the retinal eccentricity and the topological relationship with the lenticular co-explant. The Wilcoxon matched-pairs signed-rank test was used to determine if the numbers of axons differed significantly between four regions of the explants. Significantly more axons grew out from the retinal edge facing the lenticular explant than from its opposite side, indicating that the lens epithelium supports axon growth. The numbers of surviving retinal ganglion cells in culture were determined after retrograde prelabelling with a neuroanatomical tracer. The number of fluorescent ganglion cells within the retinal explants did not significantly differ between the groups (Mann-Whitney test). These findings indicate that the lens epithelium influences both the amount of axonal regeneration and the direction of growth without affecting the survival rate of retinal ganglion cells in vitro.

Expression patterns of Xenopus FGF receptor-like 1/nou-darake in early Xenopus development resemble those of planarian nou-darake and Xenopus FGF8

Fibroblast growth factors (FGFs) mediate many cell-to-cell signaling events during early development. Nou-darake (ndk), a gene encoding an FGF receptor (FGFR)-like molecule, was found to be highly and specifically expressed in the head region of the planarian Dugesia japonica, and its functional analyses provided strong molecular evidence for the existence of a brain-inducing circuit based on the FGF signaling pathway. To analyze the role of ndk during vertebrate development, we isolated the Xenopus ortholog of ndk, the vertebrate FGFR-like 1 gene (XFGFRL1). Expression of XFGFRL1/Xndk was first detected in the anterior region at the late gastrula stage and dramatically increased at the early neurula stage in an overall anterior mesendodermal region, including the prechordal plate, paraxial mesoderm, anterior endoderm, and archenteron roof. This anterior expression pattern resembles that of ndk in planarians, suggesting that the expression of FGFRL1/ndk is conserved in evolution between these two distantly diverged organisms. During the tail bud stages, XFGFRL1/Xndk expression was detected in multiple regions, including the forebrain, eyes, midbrain-hindbrain boundary, otic vesicles, visceral arches, and somites. In many of these regions, XFGFRL1/Xndk was coexpressed with XFGF8, indicating that XFGFRL1/Xndk is a member of the XFGF8 synexpression group, which includes sprouty, sef, and isthmin.

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.

Signaling through FGF receptor-2 is required for lens cell survival and for withdrawal from the cell cycle during lens fiber cell differentiation

Fibroblast growth factors (FGFs) play important roles in many aspects of development, including lens development. The lens is derived from the surface ectoderm and consists of an anterior layer of epithelial cells and elongated, terminally differentiated fiber cells that form the bulk of the tissue. FGF signaling has been implicated in lens induction, proliferation, and differentiation. To address the role of FGFs in lens development, we inactivated FGF receptor-2 (Fgfr2) using a Cre transgene that is expressed in all prospective lens cells from embryonic day 9.0. Inactivation of Fgfr2 shows that signaling through this receptor is not required for lens induction or for the proliferation of lens epithelial cells. However, Fgfr2 signaling is needed to drive lens fiber cells out of the cell cycle during their terminal differentiation. It also contributes to the normal elongation of primary lens fiber cells and to the survival of lens epithelial cells.

Can lenticular factors improve the posttrauma fate of neurons?

The intraocular lens has recently been recognized as a potential source for neuroprotective and neurite-promoting activities. The lens is ontogenetically and functionally a peculiar intraocular tissue with the unique feature of performing incomplete cellular apoptosis throughout the lifetime. The ectodermally derived epithelial cells permanently divide to produce the nuclei- and organelle-free lens fibre cells that allow for the optical transparency. The underlying extremely specific physical, biochemical, metabolic and structural mechanism lead to efficient protection from photo-oxidative stress caused by exposure to short-wavelength light. The fact that fibre cells undergo incomplete apoptosis is also of crucial importance to other cellular systems. In particular, injured nerve cells such as axotomized retinal ganglion cells may profit from the apoptosis-blocking mechanisms operating within the lens fibres. In this review we first discuss some factors involved in the lens differentiation and partial apoptosis as a basic principle of long-term survival. We then present recent experimental evidence that lenticular factors also operate outside the lens, and in particular within the retina to contribute to axonal regeneration, e.g. after a trauma. In turn, factors such as GAP-43 that were thought to be exclusively expressed within nervous tissue have now also been discovered within the lenticular tissue. Experiments of the direct confrontation of lenticular epithelial and fibre cells with regenerating ganglion cell axons in vitro are presented. It is concluded that survival factors supplied by the lens might be used to facilitate survival within neuronal tissue.

Roles of Maf family proteins in lens development

Lens provides a good model for studying developmental cues relevant to cellular and molecular interactions. Basic region/leucine zipper (bZIP) transcription factors have been found to play key roles during eye formation in various species, including human, mouse, rat, Xenopus, zebrafish, chick, and quail. Different ocular developmental anomalies associated with MAF mutation in human implicate its active role during eye development. Several members of the maf gene family with this bZIP motif participate directly in lens morphogenesis. One vital Maf protein, L-Maf, is expressed in developing lens cells of chick embryos. Its homolog recently has been detected in lens placode of Xenopus embryos and regulates expression of lens fiber-specific genes in this species. Ectopic expression of L-Maf can induce lens-specific genes in cultured retina cells and embryonic ectoderm. The dominant-negative form of L-Maf causes the suppression of crystallin expression and subsequently inhibits lens formation, indicating that L-Maf plays a central role in chick lens development.

Lens-forming competence in the epidermis ofXenopus laevis during development

In larval X. laevis the capacity to regenerate a lens under the influence of inductive factors present in the vitreous chamber is restricted to the outer cornea and pericorneal epidermis (Lentogenic Area, LA). However, in early embryos, the whole ectoderm is capable of responding to inductive factors of the larval eye forming lens cells. In a previous paper, Cannata et al. (2003) demonstrated that the persistence of lens-forming competence in the LA is the result of early signals causing lens-forming bias in the presumptive LA and of late signals from the eye causing cornea development. This paper analyzes 1) the decrease of the lens-forming capacity in ectodermal regions both near LA (head epidermis) and far from LA (flank epidermis) during development, 2) the capacity of the head epidermis and flank epidermis to respond to lens-competence promoting factors released by an eye transplanted below these epidermal regions, and 3) the eye components responsible for the promoting effect of the transplanted eye. Results were obtained by implanting fragments of ectoderm or epidermis into the vitreous chamber of host tadpoles and by evaluating the percentage of implants positive to a monoclonal antibody anti-lens. These results demonstrated that the lens-forming competence in the flank region is lost at the embryonic stage 30/31 and is weakly restored by eye transplantation; however, lens-forming competence in the head region is lost at the larval stage 48 and is strongly restored by eye transplantation. The authors hypothesize that during development the head ectoderm outside the LA is attained by low levels of the same signals that attain the LA and that these signals are responsible for the maintenance of lens-forming competence in the cornea and pericorneal epidermis of the larva. In this hypothesis, low levels of these signals slacken the decrease of the lens-forming competence in the head ectoderm and make the head epidermis much more responsive than the flank epidermis to the effect of promoting factors released by a transplanted eye. Results obtained after transplantation of eyes deprived of some components indicate that the lens and the retina are the main source of these promoting factors. The immunohistochemical detection of the FGFR-2 (bek variant) protein in the epidermis of stage 53 larvae submitted to eye transplantation at stage 46 showed that the eye transplantation increased the level of FGFR-2 protein in the head epidermis but not in the flank epidermis, indicating that the lens-forming competence in X. laevis epidermis could be related to the presence of an activated FGF receptor system in the responding tissue.

Mouse pigpen encodes a nuclear protein whose expression is developmentally regulated during craniofacial morphogenesis

Pigpen, a nuclear protein with RNA-binding motifs and a putative transcriptional activation domain (TAD), is expressed at high levels in proliferating endothelial cells and expression is down-regulated when cells adopt a quiescent or differentiated phenotype. We cloned the mouse homolog of pigpen and investigated the regulation of its expression during embryogenesis. In situ hybridization demonstrated that a broad pattern of pigpen expression became restricted during tooth formation in the mandible. In the eye, pigpen showed a spatial restriction to the more proliferating and less differentiated regions of the lens and neural retina. Expression was also restricted in the developing vibrissae, lung, and kidney, all sites where epithelial-mesenchymal interactions are vital for morphogenesis. In vitro assays, that focused on the mandible and tooth development, indicated that epithelial signals, mediated by fibroblast growth factor-8, were required to maintain pigpen expression in the mandibular mesenchyme, whereas bone morphogenetic protein-4 negatively regulated expression in that tissue during early odontogenesis. At the protein level, immunocytochemistry demonstrated that Pigpen was expressed diffusely in the cytoplasm and more concentratedly in focal granules within the nuclei of mouse embryonic cells. Lastly, CAT reporter assays showed that the N-terminus of mouse pigpen encodes an active TAD. These data suggest that mouse Pigpen may activate transcription in vivo in response to specific growth factor signals and regulate proliferation and/or differentiation events during mouse organogenesis.

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

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

What causes steroid cataracts? A review of steroid-induced posterior subcapsular cataracts

Prolonged use of glucocorticoids is a significant risk factor for the development of posterior subcapsular cataract. This places restrictions on the use of glucocorticoids in the treatment of systemic and/or ocular inflammatory conditions as well as in organ transplantation. The mechanisms responsible for the opacification are unknown and no effective treatment, other than surgical removal of the lens, is available. Difficulties in establishing suitable in vivo or in vitro models have limited research in this area. Nevertheless, several mechanisms, based on observations with other types of cataracts, have been proposed. In this review, these mechanisms are evaluated in light of the evidence available. A novel mechanism is also proposed, in which steroids do not directly act on the lens but rather affect the balance of ocular cytokines and growth factors.

Circadian variations of transforming growth factor-beta2 and basic fibroblast growth factor in the rabbit aqueous humor

PURPOSE

To compare levels of transforming growth factor-beta2 (TGF-beta2) and basic fibroblast growth factor (bFGF) in the ocular fluids of rabbits at various times of the day.

METHODS

Seventy-two young adult New Zealand albino rabbits were entrained to a daily 12-hour light (6 AM to 6 PM) and 12-hour dark cycle. Eight rabbits were sacrificed at each of 6 time points: 4 AM, 8 AM, noon, 4 PM, 8 PM, and midnight. Under an acute 24-hour constant dark condition, 8 additional rabbits were sacrificed at each of 3 time points in the accustomed light period (8 AM, noon, and 4 PM). Samples of aqueous humor and vitreous humor were collected from both eyes. Using enzyme immunoassay, samples from one eye were assayed for total TGF-beta2 and active form of TGF-beta2 and samples from the other eye were assayed for bFGF. Concentrations at various time points were analyzed using one-way ANOVA and post-hoc Bonferroni t-test for multiple comparisons.

RESULTS

Aqueous humor levels of total TGF-beta2, active TGF-beta2, and bFGF changed during the 24-hour period. Total TGF-beta2 and bFGF levels increased between 4 AM and 8 AM. Active TGF-beta2 level increased between 8 PM and midnight. Similar patterns of changes were found when considering aqueous humor samples collected under the acute constant dark condition. In the vitreous humor, total TGF-beta2, active TGF-beta2, and bFGF remained unchanged throughout the 24 hours.

CONCLUSIONS

Endogenous circadian rhythms of total TGF-beta2, active TGF-beta2, and bFGF occur in the rabbit aqueous humor. These two extracellular signaling molecules may be associated with specific physiological functions in the anterior segment according to the time of the day.

A novel role for FGF and extracellular signal-regulated kinase in gap junction-mediated intercellular communication in the lens

Gap junction-mediated intercellular coupling is higher in the equatorial region of the lens than at either pole, a property believed to be essential for lens transparency. We show that fibroblast growth factor (FGF) upregulates gap junctional intercellular dye transfer in primary cultures of embryonic chick lens cells without detectably increasing either gap junction protein (connexin) synthesis or assembly. Insulin and insulin-like growth factor 1, as potent as FGF in inducing lens cell differentiation, had no effect on gap junctions. FGF induced sustained activation of extracellular signal-regulated kinase (ERK) in lens cells, an event necessary and sufficient to increase gap junctional coupling. We also identify vitreous humor as an in vivo source of an FGF-like intercellular communication-promoting activity and show that FGF-induced ERK activation in the intact lens is higher in the equatorial region than in polar and core fibers. These findings support a model in which regional differences in FGF signaling through the ERK pathway lead to the asymmetry in gap junctional coupling required for proper lens function. Our results also identify upregulation of intercellular communication as a new function for sustained ERK activation and change the current paradigm that ERKs only negatively regulate gap junction channel activity.

Downregulated Expression of Integrin α6 by Transforming Growth Factor-β1 on Lens Epithelial Cells in Vitro

Integrins represent the main cell surface receptors that mediate cell-matrix and cell-cell interactions. They play critical roles in adhesion, migration, morphogenesis, and the differentiation of several cell types. Previous studies have demonstrated that members of the fibroblast growth factor (FGF)-2, transforming growth factor (TGF)-beta(1), and insulin growth factor (IGF)-1 play important roles in lens biology. In particularly, TGF-beta(1) appears to play a key role in extracellular matrix production, cell proliferation, and cell differentiation of lens epithelial cells. In this study we investigated the effects of FGF-2, TGF-beta(1), and IGF-1 on the modulation of integrin receptors using lens epithelial cell lines (HLE B-3 and alphaTN-4) and lens explants. We found that the expression of integrin alpha6 is downregulated by TGF-beta(1), but is not responsive to FGF-2 or IGF-1. The promoter activity of the integrin alpha6 gene decreased upon TGF-beta(1) treatment in a transient transfection assay, and flow cytometric analysis demonstrated the reduced expression of integrin alpha6 by TGF-beta(1), whereas significant changes were not observed in the level of integrin alpha6 after the addition of FGF-2. These findings suggest that the reduced expression of integrin alpha6 caused by TGF-beta(1) might play a role in the activation of the cell cycle genes required during the fiber differentiation of the lens.

FGF signaling in chick lens development

The prevailing concept has been that an FGF induces epithelial-to-fiber differentiation in the mammalian lens, whereas chick lens cells are unresponsive to FGF and are instead induced to differentiate by IGF/insulin-type factors. We show here that when treated for periods in excess of those used in previous investigations (>5 h), purified recombinant FGFs stimulate proliferation of primary cultures of embryonic chick lens epithelial cells and (at higher concentrations) expression of the fiber differentiation markers delta-crystallin and CP49. Surprisingly, upregulation of proliferation and delta-crystallin synthesis by FGF does not require activation of ERK kinases. ERK function is, however, essential for stimulation of delta-crystallin expression in response to insulin or IGF-1. Vitreous humor, the presumptive source of differentiation-promoting activity in vivo, contains a factor capable of diffusing out of the vitreous body and inducing delta-crystallin and CP49 expression in chick lens cultures. This factor binds heparin with high affinity and increases delta-crystallin expression in an ERK-insensitive manner, properties consistent with an FGF but not insulin or IGF. Our findings indicate that differentiation in the chick lens is likely to be mediated by an FGF and provide the first insights into the role of the ERK pathway in growth factor-induced signal transduction in the lens.

Particle irradiation induces FGF2 expression in normal human lens cells

Particle Irradiation Induces FGF2 Expression in Normal Human Lens Cells. Particle radiations, including both proton and helium-ion beams, have been used to successfully treat choroidal melanoma, but with the complication of radiation-induced cataract. We have investigated a role for radiation-induced changes in the expression of basic fibroblast growth factor (FGF2) gene expression as part of the mechanism(s) underlying lens cell injury associated with cataract. Normal human lens epithelial (HLE) cells were cultured in vitro on extracellular matrix (ECM) originated from bovine corneal endothelial cells. This study reports evidence for rapid but transient induction of FGF2 transcripts, an increase of between 5- and 8-fold, within 0.5 h after exposure to particle radiation, followed by another wave of increased transcription at 2-3 h postirradiation. Immunofluorescence results confirm the enhanced levels of FGF2 protein rapidly after exposure to protons or helium ions, followed by another wave of increased activity unique to helium at 6 h postirradiation. This second wave of increased immunoreactivity was not observed in the proton-irradiated samples. Total FGF2 protein analysis after helium-ion exposures shows induced expression of three FGF2 isoforms, with an increase of up to 2-fold in the 18-kDa low-molecular-weight species. Studies of the effects of protons on individual FGF2 protein isoforms are in progress. Several mechanisms involving a role for FGF2 in radiation-induced cataract are discussed.

Temporal and spatial growth patterns in the normal and cataractous human lens

This study presents a computational model of the growth of the normal human lens and the induction of spoke-like cortical cataract in the aging lens. The anterior lens is modelled as a 2-D disk with a circumferential germinative zone. Lens cortical fibre cells in the same generation cover the surface in three identical 120 deg growth wedge-shaped sectors, with centre cardinal fibres at the 90, 210 and 330 deg meridians. In the foetal lens all primary fibre cells begin to elongate simultaneously. Anterior migration is spatially asynchronous, where the centre fibre begins to move towards the anterior pole first. The fibres at the end of the sector move last in the anterior direction. Fibre elongation advanced at constant speed until the boundary of the sector is reached. Spatio-temporal asynchrony and random fluctuations were increased for the adult lens. The model foetal lens evolved Y-shaped sutures anteriorly, and an inverted Y-shaped posteriorly. Fibre length varied periodically with meridional angle. The adult lens displayed irregular growth. If clusters of germinative cells are caused to opacify the resultant opacities are predominantly spoke-shaped. The model mimics crystalline lens fibre growth to the extent of successfully evolving lens sutures. Fluctuations in lens mass are consistent with an ordered pattern of growth. Lens senescence includes a progressive loss of spatio-temporal synchrony in fibre migration from the germinative zone. Peripheral light focusing by the anterior eye is a possible explanation for the nasal predilection and cuneiform shape of age-related cortical cataract.

Expression and characterization of SPARC in human lens and in the aqueous and vitreous humors

SPARC (secreted protein, acidic and rich in cysteine) is a matricellular glycoprotein that regulates morphogenesis, cellular proliferation, and differentiation. SPARC is a critical factor in the development and maintenance of lens transparency in mice. SPARC-null mice develop lenticular opacity at an early age that progresses gradually to mature cataract. Despite the high level of homology between the mouse and human genes, little is known about SPARC in the human lens. We have studied the expression of SPARC protein in human lens and surrounding ocular tissues from normal human donors (60-70 years old). Immunohistochemical and immunoblot analyses were conducted on lens, aqueous humor, vitreous, ciliary epithelium, pigment epithelium, cornea and retina. The epithelia and capsule of the lens contained SPARC, whereas the cortical and nuclear fibers did not. In contrast, the aqueous humor and vitreous, which provide nutrients to the lens and regulate its development and function, contained significant amounts of SPARC. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of extracts of various ocular tissues revealed bands of 43 and 29 kD after disulfide bond reduction that were reactive with anti-SPARC IgG. Despite the presence of protease inhibitors during sample preparation, we observed cleavage of intact SPARC to a 29 kD fragment, a peptide reported in other tissues and attributed to endogenous proteolysis. In addition, bands of molecular mass 150 and 200 kD were present that appeared to be disulfide-bonded complexes of SPARC monomers. Human cornea, ciliary epithelium, pigment epithelium and retina also contained SPARC. The presence of SPARC in the aqueous humor and vitreous, as well as in the lens, indicates a functional importance of SPARC in adult human eye as well as in lens development.

Re-orientation and faster, directed migration of lens epithelial cells in a physiological electric field

The vertebrate lens drives current through itself in a pattern which concentrates current efflux at the lens equator. Lens epithelial cells (LECs) move into this region where they change shape and differentiate into lens fibre cells. The mechanisms underpinning these cell behaviors are unclear. We have attempted to mimic, in isolation, the effects which such electrical signals have on LEC behaviors, by culturing LECs in a physiological DC electric field (EF) similar to that in lens. Primary human (PHLECs), primary bovine (PBLECs) and a transformed human cell line (THLECs) all changed shape to lie perpendicular to the EF, the same orientation which LECs adopt with respect to the equatorial EF as they differentiate into lens fibre cells. Exposure to an EF also significantly increased the migration rate of all three LEC types. All three LECs also showed directed cell migration although, curiously, different cell types moved in different directions. PBLECs and THLECs showed voltage-dependent, anode-directed migration, with a response threshold between 100-150 mV mm(-1)and 25-50 mV mm(-1), respectively. Small sheets of THLECs also migrated anodally. By contrast PHLECs migrated cathodally with a response threshold below 100 mV mm(-1). Reversing the polarity reversed the migration direction for each cell type. These observations raise three possibilities: (1) that small electric field may be one of the cues regulating lens epithelial cell behaviors in vivo; (2) that altering the in vivo electric field by lens replacement may contribute to the aberrant migration of epithelial cells in conditions such as posterior capsule opacification and (3) that applying electric fields may be one way of controlling aberrant lens epithelial cell behaviors.

Growth factor receptor gene and protein expressions in the human lens

In this study the mRNAs encoding epidermal growth factor receptor (EGFR), basic fibroblast growth factor receptor (FGFR-2) and insulin-like growth factor receptor (IGFR-1) genes of the human normal lenses at ages varying from 0.5 to 72 years, were identified by semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR). Regulation of EGFR gene expression in the lens did not change with aging, and of FGFR-2 and IGFR-1 genes also remained unaltered up to age 53 years. However, expressions of FGFR-2 and IGFR-1 genes were decreased at ages above 60 years. EGFR, FGFR-2 and IGFR-1 proteins were detected by immunoblot analysis in the epithelial cell membranes of lens at age varying from 40 to 72 years. There was no detectable amount of EGFR protein in fiber cell membranes of the lens, and the levels of FGFR-2 and IGFR-1 proteins were much lower than those in the epithelial cell membranes. The low levels of these receptor proteins in the fiber cell membranes of lens, suggest their possible role in keeping the differentiated function of these unique transparent cells. The findings of the increased protein levels with age of EGFR with the appearance of some degradation products at age 48 years and higher, and the increased FGFR-2 protein at age 60 years and higher in the epithelial cell membranes of lens, were of interest. It appears that this could be a compensatory protective response of the lens to aging process for lifelong continuation of normal growth by proliferation and differentiation of its epithelial cells into new fiber cells in the germinative zone at the equatorial region. Thus, these results could provide a basis for further studies on growth factor receptor gene and protein regulations in the mechanism of lens aging and progression of age-related human cataract.

A forkhead gene, FoxE3, is essential for lens epithelial proliferation and closure of the lens vesicle

In the mouse mutant dysgenetic lens (dyl) the lens vesicle fails to separate from the ectoderm, causing a fusion between the lens and the cornea. Lack of a proliferating anterior lens epithelium leads to absence of secondary lens fibers and a dysplastic, cataractic lens. We report the cloning of a gene, FoxE3, encoding a forkhead/winged helix transcription factor, which is expressed in the developing lens from the start of lens placode induction and becomes restricted to the anterior proliferating cells when lens fiber differentiation begins. We show thatFoxE3 is colocalized with dyl in the mouse genome, thatdyl mice have mutations in the part of FoxE3 encoding the DNA-binding domain, and that these mutations cosegregate with thedyl phenotype. During embryonic development, the primordial lens epithelium is formed in an apparently normal way in dylmutants. However, instead of the proliferation characteristic of a normal lens epithelium, the posterior of these cells fail to divide and show signs of premature differentiation, whereas the most anterior cells are eliminated by apoptosis. This implies that FoxE3 is essential for closure of the lens vesicle and is a factor that promotes survival and proliferation, while preventing differentiation, in the lens epithelium.

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.

bFGF suppresses serum-deprivation-induced apoptosis in a human lens epithelial cell line

There is increasing evidence that basic fibroblast growth factor (bFGF) plays an important role in cell proliferation, differentiation, and survival in various systems. In the eye, although a truncated, dominant negative bFGF receptor in transgenic mice induced defective lens development and caused lens fiber cells to display characteristics of apoptosis, there is little direct evidence of the effect of bFGF on lens epithelial cell apoptosis. Our study examines the effects of bFGF on programmed cell death induced by serum deprivation using a human lens epithelial cell line. Cells supplemented with 20% fetal bovine serum were used as normal controls. Over a period of 7 days, the addition of 100 ng/ml bFGF effectively suppressed serum-deprived apoptosis. The expression of gamma-crystallin and major intrinsic protein, which are markers of lens cell differentiation, was not detected. Also there was no significant difference in cell proliferation between serum-deprived cells with or without bFGF. ICE (caspase-1) was expressed under both the conditions, but the level of expression between the two groups was not substantially different. bcl-2 and c-myc were upregulated only in bFGF-treated cells. Thus we speculate that the inhibitory effect of bFGF on apoptosis is through the upregulation of the inhibitor of apoptosis, instead of downregulation of the initiator. This effect appears to be independent of lens cell differentiation and proliferation.

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.

Overlapping effects of different members of the FGF family on lens fiber differentiation in transgenic mice

Fibroblast growth factors (FGFs), such as FGF-1, have been shown to induce differentiation of lens epithelial cells both in tissue culture and in transgenic mice. In the present study, using the alpha A-crystallin promoter, we generated transgenic mice that express different FGFs (FGF-4, FGF-7, FGF-8, FGF-9) specifically in the lens. All four FGFs induced changes in ocular development. Microphthalmic eyes were evident in transgenic mice expressing FGF-8, FGF-9 and some lines expressing FGF-4. A developmental study of the microphthalmic eyes revealed that, by embryonic day 15, expression of these FGFs induced lens epithelial cells to undergo premature fiber differentiation. In less severely affected lines expressing FGF-4 or FGF-7, the lens epithelial cells exhibited a premature exit from the cell cycle and underwent a fiber differentiation response later in development, leading to cataract formation. The responsiveness of lens cells to different FGFs indicates that these proteins stimulate the same or overlapping downstream signalling pathway(s). These overlapping effects of different FGFs on a common cell type indicate that the normal developmental roles for these genes are determined by the temporal and spatial regulation of their expression patterns. The fact that any of these FGFs can induce ocular defects and loss of lens transparency implies that it is essential for the normal eye to maintain very specific spatial control over FGF expression in order to prevent cataract induction.

Basic fibroblast growth factor (FGF-2) in mouse tooth morphogenesis

This study describes the spatio-temporal expression of basic Fibroblast growth factor (FGF-2) during odontogenesis of mouse as revealed by immunohistology. Parasagittal sections of mouse embryo head (13-18 day of gestation) containing various stages of developing tooth were incubated with a polyclonal anti-FGF-2 antibody and positive binding was evidentiated by using Streptavidin-Biotin complex-HRP system and AEC staining. We observed no FGF-2 staining at the dental lamina stage. At the bud stage slight staining is seen, limited to some epithelial cells. The intensity of the staining increases at the cap stage. In the bell stage, the stellate reticulum cells stain intensely. Later, odontoblasts and the dentin matrix stain deeply; but the epithelial cells stain faint. The extra cellular matrix of the dentin and dental papilla stain very intense but the enamel matrix is found negative. These results indicate the participation of FGF-2 in differentiation rather than in proliferation of tooth-forming cells. In particular, it appears that FGF-2 participates in odontoblast differentiation and in dentin matrix deposition. The spatio-temporally specific distribution pattern of FGF-2 in developing mouse tooth reported here emphasizes the importance of FGF-2 in mammalian odontogenesis.

Overexpression of FGF-2 modulates fiber cell differentiation and survival in the mouse lens

During mammalian embryogenesis, the ocular lens forms through a temporally and spatially regulated pattern of differentiation which is thought to be coordinated at least in part by the FGF-1 and FGF-2 members of the fibroblast growth factor (FGF) family. Previous transgenic experiments in which FGF-1 or dominant negative FGF receptors were overexpressed in the lens indicated that FGF-1 could induce differentiation while differentiated lens cells rely upon FGF signaling for their survival. In this study, we asked if the 17.5 kDa FGF-2 protein was capable of inducing differentiation of lens cells in transgenic mice. Unexpectedly, differentiation was inhibited by lens-specific expression of a transgene encoding a secreted form of the 17.5 kDa bovine FGF-2 protein under the transcriptional control of the murine alphaA-crystallin promoter (alphaAIgFGF-2 transgenic mice). To address the possibility that FGF-2 functions as a modulator of fiber cell survival, alphaAIgFGF-2 transgenic mice were crossed to transgenic mice exhibiting extensive apoptosis in the lens due to the functional inactivation of the retinoblastoma protein (alphaAE7 transgenic mice). The level of apoptosis in the lenses of double transgenic mice was substantially reduced as compared to the level in lenses from alphaAE7 only mice. These studies indicate that FGF-2 can act as a modulator of the later stages of differentiation including fiber cell survival. Additionally, they imply that control of lens development by FGFs is a complex process in which FGF-1 and FGF-2 play distinct roles.

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.

Expression of FGF-1 and FGF-2 mRNA during lens morphogenesis, differentiation and growth

PURPOSE

There is now considerable evidence that FGF is involved in lens differentiation and growth throughout life. The aim of this study was to determine potential sites of FGF production in and near the lens during morphogenesis, differentiation and growth.

METHODS

The distribution of FGF-1 and FGF-2 mRNAs was analysed in embryonic, weanling and adult rat eyes by in situ hybridization.

RESULTS

During lens morphogenesis, there was distinct expression of FGF-1, but not FGF-2, in the lens placode and retinal disc cells. Subsequently, both forms of FGF showed similar expression patterns. During lens differentiation, distinct expression of FGFs was associated with elongating primary fiber cells. From embryonic day 20 onwards, lenses showed strongest expression of FGF mRNAs in the transitional zone, where epithelial cells differentiate into fibers, with weaker expression in the anterior epithelium. Messenger RNAs for both FGFs were also localised in ocular tissues near the lens and bordering the ocular media, particularly the cornea, ciliary body, iris and neural retina.

CONCLUSIONS

These findings are consistent with the known distribution of FGF protein in the eye and implicate various ocular tissues as potential sources of FGF that may influence lens cells. Furthermore, the fact that lens cells have the potential for synthesizing FGF, together with evidence from previous studies that lens cells express FGF receptors and respond to lens-derived FGF, raises the possibility that some aspects of lens cell behaviour in situ may be influenced by autocrine mechanism(s) of FGF stimulation.

Nerve-blastema interactions induce fibroblast growth factor-1 release during limb regeneration in Pleurodeles waltl

Previous studies have shown that both fibroblast growth factor (FGF)-1 and nerves play an important function during limb regeneration, but no correlation between these two regeneration factors has yet been demonstrated. In the present study we first establish that exogenous FGF-2, a member of the FGF family that binds to the same high-affinity receptors as FGF-1, is able to stimulate both [3H]-thymidine incorporation and the mitotic index in the mesenchyme and the epidermal cells of denervated blastemas. We then use cocultures of spinal cord and blastema on heparin-coated dishes, an in vitro system mimicking the in vivo interactions during limb regeneration, to show that interactions between nerve fibers from the spinal cord and the blastema enhance the release of bioactive FGF-1. Release of this growth factor seemed to correlate with nerve fiber regeneration, as it decreased in the presence of the dipeptide Leu-Ala, known to inhibit neurite outgrowth, while the inverse dipeptide Ala-Leu was inactive. Therefore, these results support our hypothesis that the interaction between nervous tissue and blastema is permissive for the release of FGF-1, which in turn stimulates blastema cell proliferation.

In vitro lens transdifferentiation of Xenopus laevis outer cornea induced by Fibroblast Growth Factor (FGF)

It has been shown that lens regeneration from outer cornea of larval Xenopus laevis is dependent on neural retina both in vivo and in tissue culture. The isolated outer cornea cultured in the presence of bovine brain-derived acidic Fibroblast Growth Factor (aFGF) is able to reprogram the differentiation into lens fibers, although this transdifferentiative process is not coupled with the formation of a normally organized lens. The capacity of aFGF to promote lens differentiation from cornea is not linked to its mitogenic activity. The cultured corneal cells can transdifferentiate into lens fibers in the presence of aFGF when DNA replication and cell proliferation are prevented by addition of aphidicolin, a specific inhibitor of DNA polymerase in eukaryotes, to the culture medium.

Binding of FGF-1 and FGF-2 to heparan sulphate proteoglycans of the mammalian lens capsule

In the mammalian eye, FGF plays a key role in the induction of lens fibre differentiation and, in other systems, heparan sulphate proteoglycans (HSPGs) have been shown to modulate FGF activity. HSPGs were isolated from the anterior and posterior rat and bovine lens capsule and assessed in terms of their ability to bind FGF-1 and FGF-2. In the rat, at least four HSPGs were identified with molecular weights of 142, 166, 200 and approximately 250 kD, the latter species predominating. The capsule HSPGs bound both FGF-1 and FGF-2. There appeared to be little, if any, competition for binding between FGF-1 and FGF-2. The capsule contained substantial amounts of core protein, which did not bind FGF, with a higher core protein/HSPG ratio in the anterior than in the posterior capsule. This was the only major HSPG-related difference noted between anterior and posterior capsule.

Cellular and molecular features of lens differentiation: a review of recent advances

In this paper, the more recent literature pertaining to differentiation in the developing vertebrate lens is reviewed in relation to previous work. The literature reviewed reveals that the developing lens has been, and will continue to be, a useful model system for the examination of many fundamental processes occurring during embryonic development. Areas of lens development reviewed here include: the induction and early embryology of the lens; lens cell culture techniques; the role of growth factors and cytokines; the involvement of gap junctions in lens cell-cell communication; the role of cell adhesion molecules, integrins, and the extracellular matrix; the role of the cytoskeleton; the processes of programmed cell death (apoptosis) and lens fibre cell denucleation; the involvement of Pax and Homeobox genes; and crystallin gene regulation. Finally, some speculation is provided as to possible directions for further research in lens development.

The effect of scatter laser photocoagulation on intravitreal levels of growth factors in the miniature pig

PURPOSE

To characterise changes in intravitreal growth factor profiles following retinal photocoagulation in the miniature pig.

METHODS

Miniature pig eyes underwent scatter photocoagulation by either diode infrared or emerald green laser. Animals were sacrificed at various times (up to 42 days) post-laser. The eyes were than removed and vitreous samples analysed for basic fibroblast growth factor, insulin-like growth factor-I and epidermal growth factor by radioimmunoassay, transforming growth factor-beta 2 by ELISA and insulin-like growth factor binding proteins using Western ligand blotting.

RESULTS

Vitreous transforming growth factor-beta 2 levels were decreased at 1 h post diode laser and at 4 and 7 days post emerald laser but returned to normal by 21 and 42 days respectively. Vitreous insulin-like growth factor-I levels increased at 4 and 7 days post diode and emerald laser respectively but returned to normal by 21 days. Insulin-like growth factor Western ligand blotting demonstrated that a 34 kDa insulin-like growth factor binding protein was predominant in the pig vitreous; the levels of this binding protein followed an identical trend to those observed for insulin-like growth factor-I. No changes in vitreous levels of either basic fibroblast growth factor or epidermal growth factor were observed following laser treatment.

CONCLUSIONS

Our results demonstrate a significant shift in the balance of intravitreal growth factors following retinal laser photocoagulation. Such changes may be pertinent to the regression of preretinal new vessels after laser photocoagulation.

Expression of a truncated FGF receptor results in defective lens development in transgenic mice

Members of the fibroblast growth factor (FGF) family are thought to initiate biological responses through the activation of cell surface receptors which must dimerize to transmit an intracellular signal. Mammalian lens epithelial cells respond to exogenous extracellular FGF, either in tissue culture or in transgenic mice, by initiating fiber cell differentiation. The role of FGF signalling in normal lens development was evaluated by lens-specific synthesis of a kinase-deficient FGF receptor type I (FGFR1) in transgenic mice. This truncated FGF receptor is thought to act as a dominant negative protein by heterodimerization with endogenous FGF receptors. The presence of transgenic mRNA in the lens was confirmed by in situ hybridization and by polymerase chain reaction amplification of reverse transcribed lens RNA (RT-PCR). The presence of transgenic protein was determined by Western blotting with antibodies to an extracellular domain of FGFR1. Three of four transgenic families expressing the truncated FGF receptor exhibited lens defects ranging from cataracts to severe microphthalmia. While the microphthalmic lenses displayed a normal pattern of differentiation-specific crystallin expression, the lens epithelial cells were reduced in number and the lens fiber cells displayed characteristics consistent with the induction of apoptosis. Our results support the view that FGF receptor signalling plays an essential role in normal lens biology.

Presence of beta-citryl-L-glutamic acid in the lens: its possible role in the differentiation of lens epithelial cells into fiber cells

The beta-CG concentration in the chicken brain was high during embryonic development and decreased rapidly to a lower level close to hatching, while the concentration in the eyeball which was also high during the embryonic life retained a fairly high level after hatching. The distribution of beta-CG in the bovine eye was determined. About 95% of total beta-CG content in the whole eye was localized in the lens. However, the distribution of beta-CG in the eye varied depending on species. beta-CG was exclusively localized in the lens in the eyes of fish and mammals, but distributed in both lens and retina in frogs. The molecule was localized in the retina rather than the lens in the chicken eye, although the concentrations was extremely low compared to those in the mammalian, amphibian and fish eyes. It was found that beta-CG is present ubiquitously in the lens or retina in various species. The distribution of beta-CG in the bovine lens was determined in the three cortex regions and nucleus. beta-CG was present at the highest concentration in the equatorial cortex, at a moderate concentration in the posterior and anterior cortex, and at the lowest concentration in the nucleus. Similar distribution patterns were also found in the rabbit and rat lens. When embryonic chick lens epithelial cells were cultured in the presence of fetal calf serum, the cells elongated, differentiated into fiber cells and formed lentoid bodies. The cells of lentoid bodies were stained strongly by the anti-beta-CG antibody, while cells around the structures were not. In addition, the beta-CG content in the lenses from the galactose cataractous rat decreased to about 20-30% of that in the normal lens. These findings suggest that beta-CG may play a role in the differentiation of epithelial cells into fiber cells.

Extracellular FGF-1 acts as a lens differentiation factor in transgenic mice

The vertebrate ocular lens undergoes a spatially defined pattern of differentiation which may be regulated by the ocular distribution of proteins from the fibroblast growth factor (FGF) family. The ability of altered FGF-1 (acidic FGF) distribution to disrupt the normal pattern of lens differentiation was evaluated by the production of transgenic mice which express FGF-1 under the control of the lens-specific alpha A-crystallin promoter. Since FGF-1 lacks a classical signal peptide consensus sequence, transgenic mice were also produced with a chimeric construct containing the signal peptide sequence of the FGF-4 gene fused in frame to the coding sequences of the FGF-1 cDNA in order to obtain extracellular expression of the transgene. The presence of transgenic mRNA and protein was confirmed by in situ hybridization, Western analysis and immunohistochemistry. The ocular histology of newborn and young adult transgenic mice expressing FGF-1 without a signal peptide appeared normal. In contrast, mice expressing secreted FGF-1 exhibited lens abnormalities including the elongation of anterior epithelial cells. Epithelial cell elongation was accompanied by expression of the fiber cell differentiation marker, beta-crystallin. These observations provide an in vivo demonstration that FGF-1 can induce anterior lens epithelial cells to express characteristics consistent with the onset of fiber cell differentiation. The transgenic induction of differentiation confirms that normal lens morphology reflects an asymmetric distribution of inductive factors within the eye.

Gene regulation and differentiation in vertebrate ocular tissues

Molecular biological techniques have contributed greatly to the study of vertebrate ocular tissues. The specification of ocular tissues has been shown to be closely related to the expression of transcription factors encoded by genes such as Pax6 and microphthalmia. Lens-specific expression of the delta 1-crystallin gene is controlled by factors, such as delta EF1, binding to its enhancer sequences. Retinal activity of the glucocorticoid hormone receptor is regulated by its binding with another transcription factor. Degeneration of photoreceptors in a retinal disease, retinitis pigmentosa, can be caused by the introduction of a mutated opsin gene into mice. In addition, the process of transdifferentiation in ocular tissues has been described at the level of gene expression.