A role for Hippo/YAP-signaling in FGF-induced lens epithelial cell proliferation and fibre differentiation

Recent studies indicate an important role for the transcriptional co-activator Yes-associated protein (YAP), and its regulatory pathway Hippo, in controlling cell growth and fate during lens development; however, the exogenous factors that promote this pathway are yet to be identified. Given that fibroblast growth factor (FGF)-signaling is an established regulator of lens cell behavior, the current study investigates the relationship between this pathway and Hippo/YAP-signaling during lens cell proliferation and fibre differentiation. Rat lens epithelial explants were cultured with FGF2 to induce epithelial cell proliferation or fibre differentiation. Immunolabeling methods were used to detect the expression of Hippo-signaling components, Total and Phosphorylated YAP, as well as fibre cell markers, Prox-1 and β-crystallin. FGF-induced lens cell proliferation was associated with a strong nuclear localisation of Total-YAP and low-level immuno-staining for phosphorylated-YAP. FGF-induced lens fibre differentiation was associated with a significant increase in cytoplasmic phosphorylated YAP (inactive state) and enhanced expression of core Hippo-signaling components. Inhibition of YAP with Verteporfin suppressed FGF-induced lens cell proliferation and ablated cell elongation during lens fibre differentiation. Inhibition of either FGFR- or MEK/ERK-signaling suppressed FGF-promoted YAP nuclear translocation. Here we propose that FGF promotes Hippo/YAP-signaling during lens cell proliferation and differentiation, with FGF-induced nuclear-YAP expression playing an essential role in promoting the proliferation of lens epithelial cells. An FGF-induced switch from proliferation to differentiation, hence regulation of lens growth, may play a key role in mediating Hippo suppression of YAP transcriptional activity.

Fibrosis in the lens. Sprouty regulation of TGFβ-signaling prevents lens EMT leading to cataract

Cataract is a common age-related condition that is caused by progressive clouding of the normally clear lens. Cataract can be effectively treated by surgery; however, like any surgery, there can be complications and the development of a secondary cataract, known as posterior capsule opacification (PCO), is the most common. PCO is caused by aberrant growth of lens epithelial cells that are left behind in the capsular bag after surgical removal of the fiber mass. An epithelial-to-mesenchymal transition (EMT) is central to fibrotic PCO and forms of fibrotic cataract, including anterior/posterior polar cataracts. Transforming growth factor β (TGFβ) has been shown to induce lens EMT and consequently research has focused on identifying ways of blocking its action. Intriguingly, recent studies in animal models have shown that EMT and cataract developed when a class of negative-feedback regulators, Sprouty (Spry)1 and Spry2, were conditionally deleted from the lens. Members of the Spry family act as general antagonists of the receptor tyrosine kinase (RTK)-mediated MAPK signaling pathway that is involved in many physiological and developmental processes. As the ERK/MAPK signaling pathway is a well established target of Spry proteins, and overexpression of Spry can block aberrant TGFβ-Smad signaling responsible for EMT and anterior subcapsular cataract, this indicates a role for the ERK/MAPK pathway in TGFβ-induced EMT. Given this and other supporting evidence, a case is made for focusing on RTK antagonists, such as Spry, for cataract prevention. In addition, and looking to the future, this review also looks at possibilities for supplanting EMT with normal fiber differentiation and thereby promoting lens regenerative processes after cataract surgery. Whilst it is now known that the epithelial to fiber differentiation process is driven by FGF, little is known about factors that coordinate the precise assembly of fibers into a functional lens. However, recent research provides key insights into an FGF-activated mechanism intrinsic to the lens that involves interactions between the Wnt-Frizzled and Jagged/Notch signaling pathways. This reciprocal epithelial-fiber cell interaction appears to be critical for the assembly and maintenance of the highly ordered three-dimensional architecture that is central to lens function. This information is fundamental to defining the specific conditions and stimuli needed to recapitulate developmental programs and promote regeneration of lens structure and function after cataract surgery.

Interactions between lens epithelial and fiber cells reveal an intrinsic self-assembly mechanism

How tissues and organs develop and maintain their characteristic three-dimensional cellular architecture is often a poorly understood part of their developmental program; yet, as is clearly the case for the eye lens, precise regulation of these features can be critical for function. During lens morphogenesis cells become organized into a polarized, spheroidal structure with a monolayer of epithelial cells overlying the apical tips of elongated fiber cells. Epithelial cells proliferate and progeny that shift below the lens equator differentiate into new fibers that are progressively added to the fiber mass. It is now known that FGF induces epithelial to fiber differentiation; however, it is not fully understood how these two forms of cells assemble into their characteristic polarized arrangement. Here we show that in FGF-treated epithelial explants, elongating fibers become polarized/oriented towards islands of epithelial cells and mimic their polarized arrangement in vivo. Epithelial explants secrete Wnt5 into the culture medium and we show that Wnt5 can promote directed behavior of lens cells. We also show that these explants replicate aspects of the Notch/Jagged signaling activity that has been shown to regulate proliferation of epithelial cells in vivo. Thus, our in vitro study identifies a novel mechanism, intrinsic to the two forms of lens cells, that facilitates self-assembly into the polarized arrangement characteristic of the lens in vivo. In this way the lens, with its relatively simple cellular composition, serves as a useful model to highlight the importance of such intrinsic self-assembly mechanisms in tissue developmental and regenerative processes.

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.

TGFbeta promotes Wnt expression during cataract development

TGFbeta induces lens epithelial cells to undergo epithelial mesenchymal transition (EMT) and many changes with characteristics of fibrosis including posterior capsular opacification (PCO). Consequently much effort is directed at trying to block the damaging effects of TGFbeta in the lens. To do this effectively it is important to know the key signaling pathways regulated by TGFbeta that lead to EMT and PCO. Given that Wnt signaling is involved in TGFbeta-induced EMT in other systems, this study set out to determine if Wnt signaling has a role in regulating this process in the lens. Using RT-PCR, in situ hybridization and immunolocalization this study clearly shows that Wnts 5a, 5b, 7b, 8a, 8b and their Frizzled receptors are upregulated in association with TGFbeta-induced EMT and cataract development. Both rat in vitro and mouse in vivo cataract models show similar profiles for the Wnt and Frizzled mRNAs and proteins that were assessed. Currently it is not clear if the canonical beta-catenin/TCF signaling pathway, or a non-canonical pathway, is activated in this context. Overall, the results from the current study indicate that Wnt signaling is involved in TGFbeta-induced EMT and development of fibrotic plaques in the lens.

Lithium stabilizes the polarized lens epithelial phenotype and inhibits proliferation, migration, and epithelial mesenchymal transition

Posterior capsule opacification (PCO) is a common complication of cataract surgery caused by epithelial mesenchymal transition (EMT) and aberrant lens cell growth. One path to prevention depends on maintaining the quiescent lens epithelial phenotype. Here we report that lithium chloride (LiCl) is a potent stabilizer of the lens epithelial phenotype. In lens epithelial explants (controls), at low cell density, cells readily depolarized, spread out, and proliferated. By contrast, in the presence of LiCl, cells did not spread out or exhibit migratory behaviour. Using concentrations of 1-30 mM LiCl we also showed that cell proliferation is inhibited in a dose-dependent manner. Confocal microscopy and immunohistochemistry for ZO-1 and E-cadherin showed that LiCl treatment maintained tight junctions at the apical margins of cells. Taken together with measurements of cell heights, this showed that the cells in LiCl-treated explants maintained the apical baso-lateral polarity and cobblestone-like packing that is characteristic of lens epithelial cells in vivo. Significantly, the effects of LiCl also extended to blocking the potent EMT/cataract-promoting effects of transforming growth factor beta (TGFbeta) on lens epithelial cells. In TGFbeta-treated explants, cells progressively dissociated from one another, taking on various elongated spindle shapes and strongly expressing alpha-smooth muscle actin (alpha-SMA). These features are characteristic of PCO. In both rat and human capsulorhexis explants, LiCl treatment effectively blocked the accumulation of alpha-SMA and maintained the cells in a polarized, adherent, cobblestone-packed monolayer. These findings highlight the feasibility of applying molecular strategies to stabilize lens epithelial cells and prevent aberrant differentiation and growth that leads to cataract.

A role for Wnt/planar cell polarity signaling during lens fiber cell differentiation?

Wnt signaling through frizzled (Fz) receptors plays key roles in just about every developmental system that has been studied. Several Wnt-Fz signaling pathways have been identified including the Wnt/planar cell polarity (PCP) pathway. PCP signaling is crucial for many developmental processes that require major cytoskeletal rearrangements. Downstream of Fz, PCP signaling is thought to involve the GTPases, Rho, Rac and Cdc42 and regulation of the JNK cascade. Here we report on the localization of these GTPases and JNK in the lens and assess their involvement in the cytoskeletal reorganisation that is a key element of FGF-induced lens fiber cell differentiation.

Transforming growth factor-beta-induced epithelial-mesenchymal transition in the lens: a model for cataract formation

The vertebrate lens has a distinct polarity and structure that are regulated by growth factors resident in the ocular media. Fibroblast growth factors, in concert with other growth factors, are key regulators of lens fiber cell differentiation. While members of the transforming growth factor (TGFbeta) superfamily have also been implicated to play a role in lens fiber differentiation, inappropriate TGFbeta signaling in the anterior lens epithelial cells results in an epithelial-mesenchymal transition (EMT) that bears morphological and molecular resemblance to forms of human cataract, including anterior subcapsular (ASC) and posterior capsule opacification (PCO; also known as secondary cataract or after-cataract), which occurs after cataract surgery. Numerous in vitro and in vivo studies indicate that this TGFbeta-induced EMT is part of a wound healing response in lens epithelial cells and is characterized by induced expression of numerous extracellular matrix proteins (laminin, collagens I, III, tenascin, fibronectin, proteoglycans), intermediate filaments (desmin, alpha-smooth muscle actin) and various integrins (alpha2, alpha5, alpha7B), as well as the loss of epithelial genes [Pax6, Cx43, CP49, alpha-crystallin, E-cadherin, zonula occludens-1 protein (ZO-1)]. The signaling pathways involved in initiating the EMT seem to primarily involve the Smad-dependent pathway, whereby TGFbeta binding to specific high affinity cell surface receptors activates the receptor-Smad/Smad4 complex. Recent studies implicate other factors [such as fibroblast growth factor (FGFs), hepatocyte growth factor, integrins], present in the lens and ocular environment, in the pathogenesis of ASC and PCO. For example, FGF signaling can augment many of the effects of TGFbeta, and integrin signaling, possibly via ILK, appears to mediate some of the morphological features of EMT initiated by TGFbeta. Increasing attention is now being directed at the network of signaling pathways that effect the EMT in lens epithelial cells, with the aim of identifying potential therapeutic targets to inhibit cataract, particularly PCO, which remains a significant clinical problem in ophthalmology.

Deregulation of lens epithelial cell proliferation and differentiation during the development of TGFbeta-induced anterior subcapsular cataract

Normal lens development and growth is dependent on the tight spatial and temporal regulation of lens cell proliferation and fiber cell differentiation. The present study reports that these same cellular processes contribute to lens pathology as they become deregulated in the process of anterior subcapsular cataract development in a transgenic mouse model. During the formation and growth of transforming growth factor (TGF)beta-induced subcapsular plaques, lens epithelial cells lose key phenotypic markers including E-cadherin and connexin 43, they multilayer and subsequently differentiate into myofibroblastic and/or fiber-like cells. Growth of the subcapsular plaques in the transgenic mouse is sustained by an ordered process of cell proliferation, exit from the cell cycle and differentiation. As reiterating ordered growth and differentiation patterns is atypical of the direct effects of TGFbeta on lens cells in vitro, we propose that other growth factors in the eye, namely fibroblast growth factor, may also play a role in the establishment and regulation of the key cellular processes leading to lens pathology. Obtaining a better understanding of the molecular aspects and cellular dynamics of cataract formation and growth is central to devising strategies for slowing or preventing this disease.

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.

Spatial and temporal expression of Wnt and Dickkopf genes during murine lens development

Recent studies indicate a role for Wnt signalling in regulating lens cell differentiation (Stump et al., 2003). To further our understanding of this, we investigated the expression patterns of Wnts and Wnt signalling regulators, the Dickkopfs (Dkks), during murine lens development. In situ hybridisation showed that Wnt5a, Wnt5b, Wnt7a, Wnt7b, Wnt8a and Wnt8b genes are expressed throughout the early lens primordia. At embryonic day 14.5 (E14.5), Wnt5a, Wnt5b, Wnt7a, Wnt8a and Wnt8b are reduced in the primary fibres, whereas Wnt7b remains strongly expressed. This trend persists up to E15.5. At later embryonic stages, Wnt expression is predominantly localised to the epithelium and elongating cells at the lens equator. As fibre differentiation progresses, Wnt expression becomes undetectable in the cells of the lens cortex. The one exception is Wnt7b, which continues to be weakly expressed in cortical fibres. This pattern of expression continues through to early postnatal stages. However, by postnatal day 21 (P21), expression of all Wnts is distinctly weaker in the central lens epithelium compared with the equatorial region. This is most notable for Wnt5a, which is barely detectable in the central lens epithelium at P21. Dkk1, Dkk2 and Dkk3 have similar patterns of expression to each other and to the majority of the Wnts during lens development. This study shows that multiple Wnt and Dkk genes are expressed during lens development. Expression is predominantly in the epithelial compartment but is also associated, particularly in the case of Wnt7b, with early events in fibre differentiation.

042.The role of TGF-β in normal and pathological lens development

How the lens develops its highly ordered architecture and growth patterns is a major question in developmental biology. During embryogenesis, cells in the anterior and posterior segments of the lens vesicle, differentiate into the epithelial and fibre cells, respectively. Our research has aimed to identify the molecules and mechanisms that regulate the divergent fates of lens cells. We have studied the roles of various growth factors in regulating lens cell fates using rat lens epithelial explant cultures and transgenic and mutant mouse models. Our research has shown that members of the FGF growth factor family are key initiators of lens fibre differentiation in mammals and there is now compelling evidence that a gradient of FGF in the eye controls lens polarity and growth patterns. Recent evidence also supports a role for TGF-β signalling in this process and indicates that a cascade of growth factor signalling may be required for normal fibre differentiation. Less is known about the anterior segment; however, our recent studies point to an important role for the Wnt growth factor family in epithelial differentiation. Growth factor signalling can also cause pathological changes; e.g. TGF-β can destabilise the normal epithelial phenotype and induce aberrant growth and differentiation that mimics the epithelial-mesenchymal transition characteristic of some forms of cataract. These studies highlight the importance of growth factor signalling in regulating the ordered growth and differentiation of the lens. It is also clear that the bioavailabity of some growth factors needs to be tightly regulated so that they act in the appropriate cellular compartment. Some cataracts may be a consequence of disturbed growth factor, particularly TGF-β, regulatory mechanisms.

TGFbeta-Smad signalling in postoperative human lens epithelial cells

AIMS

To localise Smads3/4 proteins in lens epithelial cells (LECs) of fresh and postoperative human specimens. Smads3/4 are involved in signal transduction between transforming growth factor beta (TGFbeta) cell surface receptors and gene promoters. Nuclear localisation of Smads indicates achievement of endogenous TGFbeta signalling in cells.

METHODS

Three circular sections of the anterior capsule, one lens, and 17 capsules undergoing postoperative healing were studied. Immunohistochemistry was performed for Smads3/4 in paraffin sections of the specimens. The effect of exogenous TGFbeta2 on Smad3 subcellular localisation was examined in explant cultures of extracted human anterior lens epithelium.

RESULTS

The cytoplasm, but not the nuclei, of LECs of uninjured lenses was immunoreactive for Smads3/4. In contrast, nuclear immunoreactivity for Smads3/4 was detected in LECs during capsular healing. Nuclei positive for Smads3/4 were observed in monolayered LECs adjacent to the regenerated lens fibres of Sommerring's ring. Interestingly, the nuclei of LECs that were somewhat elongated, and appeared to be differentiating into fibre-like cells, were negative for Smads3/4. Fibroblast-like, spindle-shaped lens cells with nuclear immunoreactivity for nuclear Smads3/4 were occasionally observed in the extracellular matrix accumulated in capsular opacification. Exogenous TGFbeta induced nuclear translocation of Smad3 in LECs of anterior capsule specimens in explant culture.

CONCLUSIONS

This is consistent with TGFbeta induced Smad signalling being involved in regulating the behaviour of LECs during wound healing after cataract surgery.

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.

Requirement for TGFβ receptor signaling during terminal lens fiber differentiation

Several families of growth factors have been identified as regulators of cell fate in the developing lens. Members of the fibroblast growth factor family are potent inducers of lens fiber differentiation. Members of the transforming growth factor β (TGFβ) family, particularly bone morphogenetic proteins, have also been implicated in various stages of lens and ocular development, including lens induction and lens placode formation. However, at later stages of lens development, TGFβ family members have been shown to induce pathological changes in lens epithelial cells similar to those seen in forms of human subcapsular cataract. Previous studies have shown that type I and type II TGFβ receptors, in addition to being expressed in the epithelium, are also expressed in patterns consistent with a role in lens fiber differentiation. In this study we have investigated the consequences of disrupting TGFβ signaling during lens fiber differentiation by using the mouse αΑ-crystallin promoter to overexpress mutant (kinase deficient), dominant-negative forms of either type I or type II TGFβ receptors in the lens fibers of transgenic mice. Mice expressing these transgenes had pronounced bilateral nuclear cataracts. The phenotype was characterized by attenuated lens fiber elongation in the cortex and disruption of fiber differentiation, culminating in fiber cell apoptosis and degeneration in the lens nucleus. Inhibition of TGFβ signaling resulted in altered expression patterns of the fiber-specific proteins, α-crystallin, filensin, phakinin and MIP. In addition, in an in vitro assay of cell migration, explanted lens cells from transgenic mice showed impaired migration on laminin and a lack of actin filament assembly, compared with cells from wild-type mice. These results indicate that TGFβ signaling is a key event during fiber differentiation and is required for completion of terminal differentiation.

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.

Smad translocation and growth suppression in lens epithelial cells by endogenous TGFbeta2 during wound repair

To determine whether endogenous TGFbeta affects lens epithelial cells during repair after an anterior capsule injury in mice, we studied translocation of Smad proteins, which carry the TGFbeta signal from cell surface receptors to promoters in nuclei. We immunolocalized Smads in murine lenses at intervals up to 8 weeks following capsular injury. Effects of injecting TGFbeta neutralizing antibodies on Smad4 location and cell proliferation were examined at 24 hr after injury. Finally, we examined whether exogenous TGFbeta2 induced Smad nuclear translocation in murine lenses in organ culture. Cell proliferation was quantitated by 5-bromo-2'-deoxyuridine (BrdU) labelling. In uninjured lenses, Smads were located in the cytoplasm. In injured lenses, nuclear localization of Smads was observed in cells next to the capsular break from 8 to 24 hr after the injury, and was observed peripheral to the break at 48 hr. Nuclear Smads then continued to be observed occasionally in a minority of cells. Injection of antibodies neutralizing TGFbeta2, but not TGFbeta1 or TGFbeta3, inhibited Smad4 nuclear translocation and resulted in the appearance of BrdU-positive anterior epithelial cells. With the lenses in culture, transient nuclear localization of Smads occurred between 3 and 24 hr in response to continuous exposure to TGFbeta2. No nuclear translocation was seen at 48 hr. Endogenous TGFbeta2 affects lens cells during wound repair after anterior capsule injury, inhibiting lens cell proliferation during the early phase. Nuclear translocation of Smads in lens epithelial cells is transient even with continuous exposure to TGFbeta2.

Susceptibility to TGFbeta2-induced cataract increases with aging in the rat

PURPOSE

Cataract is the most common cause of blindness in the world today, and yet there is no generally accepted treatment other than surgical intervention. Studies in rodent models designed to increase understanding of the molecular basis of cataract have shown that transforming growth factor (TGF)-beta induces morphologic and molecular changes similar to those associated with some forms of human cataract. Because aging is the most widely recognized risk factor for cataract, it is important that any animal model be examined in this context. This was a study of the effects of aging on susceptibility to TGFbeta-induced cataract.

METHODS

Lenses from weanling, adult, and senile rats were cultured in defined serum-free medium with a range of concentrations of TGFbeta2. The lenses were cultured for up to 7 days, photographed daily, fixed, and prepared for histology and immunolocalization. Opacification was quantified by image analysis.

RESULTS

Lenses from weanling, adult, and senile rats all underwent similar cataractous changes when exposed to TGFbeta. This included opacification, the formation of anterior subcapsular plaques, and accumulation of type I collagen and alpha-smooth muscle actin. Lenses from adult and senile animals, however, were generally more adversely affected by TGFbeta than lenses from weanlings. This study also showed that a low dose of TGFbeta administered over a prolonged period had an effect similar to that of a higher dose administered over a shorter period.

CONCLUSIONS

An elevation of TGFbeta activity, either acute or chronic, and/or an age-related increase in lens cell susceptibility to TGFbeta may be triggering factors in the etiology of certain forms of cataract.

Expression of Crim1 during murine ocular development

Crim1 (cysteine-rich motor neuron 1), a novel gene encoding a putative transmembrane protein, has recently been isolated and characterized (Kolle, G., Georgas, K., Holmes, G.P., Little, M.H., Yamada, T., 2000. CRIM1, a novel gene encoding a cysteine-rich repeat protein, is developmentally regulated and implicated in vertebrate CNS development and organogenesis. Mech. Dev. 90, 181-193). Crim1 contains an IGF-binding protein motif and multiple cysteine-rich repeats, analogous to those of chordin and short gastrulation (sog) proteins that associate with TGFbeta superfamily members, namely Bone Morphogenic Protein (BMP). High levels of Crim1 have been detected in the brain, spinal chord and lens. As members of the IGF and TGFbeta growth factor families have been shown to influence the behaviour of lens cells (Chamberlain, C.G., McAvoy, J. W., 1997. Fibre differentiation and polarity in the mammalian lens: a key role for FGF. Prog. Ret. Eye Res. 16, 443-478; de Iongh R.U., Lovicu, F.J., Overbeek, P.A., Schneider, M.D., McAvoy J.W., 1999. TGF-beta signalling is essential for terminal differentiation of lens fibre cells. Invest. Ophthalmol. Vis. Sci. 40, S561), to further understand the role of Crim1 in the lens, its expression during ocular morphogenesis and growth is investigated. Using in situ hybridisation, the expression patterns of Crim1 are determined in murine eyes from embryonic day 9.5 through to postnatal day 21. Low levels of transcripts for Crim1 are first detected in the lens placode. By the lens pit stage, Crim1 is markedly upregulated with high levels persisting throughout embryonic and foetal development. Crim1 is expressed in both lens epithelial and fibre cells. As lens fibres mature in the nucleus, Crim1 is downregulated but strong expression is maintained in the lens epithelium and in the young fibre cells of the lens cortex. Crim1 is also detected in other developing ocular tissues including corneal and conjunctival epithelia, corneal endothelium, retinal pigmented epithelium, ciliary and iridial retinae and ganglion cells. During postnatal development Crim1 expression is restricted to the lens, with strongest expression in the epithelium and in the early differentiating secondary fibres. Thus, strong expression of Crim1 is a distinctive feature of the lens during morphogenesis and postnatal growth.

Inhibition of FGF-induced alphaA-crystallin promoter activity in lens epithelial explants by TGFbeta

PURPOSE

Fibroblast growth factor (FGF) plays a key role in normal lens biology, and recent studies suggest that transforming growth factor (TGF)-beta is involved in the origin of certain forms of cataract. In the current study, the effects of FGF and TGFbeta on alphaA-crystallin promoter activity were investigated.

METHODS

Rat lens epithelial explants were cultured with or without growth factors after transfecting with the firefly luciferase reporter gene driven by either the mouse alphaA-crystallin promoter region or a control simian virus (SV)40 promoter.

RESULTS

FGF-2, at a concentration that induced lens fiber differentiation, strongly stimulated alphaA-crystallin promoter activity in explants at 3 to 4 days of culture, whereas SV40 promoter control specimens showed no comparable increase. At lower concentrations of FGF, sufficient to induce cell proliferation but not differentiation, there was only a slight increase in alphaA-crystallin promoter activity. Stimulation of alphaA-crystallin promoter activity induced by the fiber-differentiating concentration of FGF was virtually abolished by as little as 25 pg/ml TGFbeta2, but the onset of fiber-specific beta-crystallin accumulation was not prevented at this concentration. Phase-contrast microscopy revealed overt cataractous changes only at concentrations of TGFbeta more than 25 pg/ml.

CONCLUSIONS

The stimulation of alphaA-crystallin promoter activity by FGF is consistent with its role in inducing accumulation of crystallins in explants. The blocking effect of TGFbeta on this process, even at a concentration too low to induce obvious pathologic changes, indicates the potential for TGFbeta to disturb alphaA-crystallin gene expression during early fiber differentiation.

Intravitreal injection of TGFbeta induces cataract in rats

PURPOSE

In a previous study, it was determined that TGFbeta induces cataractous changes in the rat lens in vitro. The purpose of the present study was to determine whether the introduction of biologically active TGFbeta into the vitreous stimulates cataractous changes in the rat lens in situ.

METHODS

TGFbeta was injected into the vitreous of the left eye of anesthetized adult male Wistar rats. The right eye received sterile vehicle as a control. Three to four months after injection, animals were killed, and lenses were enucleated and examined for cataractous changes.

RESULTS

All lenses from control eyes remained transparent and maintained normal cellular architecture throughout. In contrast, lenses from TGFbeta-injected eyes displayed cloudiness in the cortex. In some lenses, distinct opacities were also apparent at the equator and extending some distance toward the anterior and posterior poles. Histologically, the opacities corresponded to subcapsular plaques containing aberrant cells and accumulations of extracellular matrix. In addition, cortical fibers in the anterior and posterior of all lenses displayed variable degrees of swelling, and many retained their nuclei. In some regions, the fiber cells appeared to have degenerated to form large homogeneous areas. The cellular architecture of the equator of these lenses was also disrupted and, in the most severe case, no bow zone was apparent with nucleated cells extending to the posterior pole.

CONCLUSION

The introduction of active TGFbeta into the vitreous induced lenses to undergo cataractous changes. In addition to the TGFbeta-induced changes in the epithelium that were reported previously, cataractous changes observed in this study also involved the lens fiber cells and resembled changes observed in human posterior subcapsular and cortical cataracts.

Spatial and temporal expression of p57(KIP2) during murine lens development

The expression patterns of p57(KIP2), an important cyclin-dependent kinase inhibitor in the lens, is investigated. This study shows that the expression of p57 mRNA throughout lens morphogenesis and growth correlates with lens cell withdrawal from the cell cycle (shown by changing patterns of BrdU incorporation) and the onset of lens fibre differentiation (shown by beta-crystallin expression). p57 expression at the early stages of fibre differentiation make it a useful marker for the initiation of this process.

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.

Ubiquitin-dependent pathway is up-regulated in differentiating lens cells

The mammalian eye lens is composed of two distinct types of cells, epithelial cells and fiber cells. The fiber cells are generated throughout life via continuous differentiation of epithelial cells. Differentiation of lens cells involves dramatic changes in cellular components including altered activity of the ubiquitin dependent pathway. The concentration of high mass ubiquitin conjugates in the mitotically active-, differentiating-equatorial epithelial cells was 5-10 fold higher than that observed in mitotically quiescent, non-differentiated, central epithelial cells, even though there was a significant dilution of non-crystallin proteins due to an increase in level of crystallins in the differentiating cells. Similar observations were made when differentiation was modeled by exposure of lens epithelial explants to bFGF in culture. Activities of ubiquitin-activating enzyme (E1) and ubiquitin-conjugating enzymes (E2s) in the differentiating equatorial epithelial cells were also up to 100% higher than those noted in non-differentiated central epithelial cells and E1 appears to be rate controlling for ubiquitinylation. Consistent with the higher concentrations of high mass ubiquitin conjugates, there was a trend of enhanced ability to execute ATP-dependent protein degradation in the differentiating equatorial epithelial cells as compared with degradation in the non-differentiated central epithelial cells. These data indicate that the ubiquitin dependent pathway is up-regulated during differentiation of lens cells. In the differentiated fibers, the concentration of high mass ubiquitin conjugates and relative activities of E1 and E2s were 50% lower than in the non-differentiated central epithelial cells. In comparison, the concentration of the 110 kDa E1 was unchanged in differentiated fibers. However, if the factor of dilution by the significant increase in the level of crystallins was taken into account, the level or activities of the components of ubiquitin pathway in the differentiated cells was higher than the level noted in non-differentiated cells. These data indicate that, as compared with other non-crystallin proteins, there is differential stabilization and/or synthesis of the 110 kDa E1 and some other components of the ubiquitin dependent pathway in differentiated fibers.

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.

Differential cataractogenic potency of TGF-beta1, -beta2, and -beta3 and their expression in the postnatal rat eye

PURPOSE

Transforming growth factor-beta has been shown to induce cataractous changes in rat lenses. This study assesses the relative cataractogenic potential of TGF-beta1, TGF-beta2, and TGF-beta3 and their expression patterns in the rat eye.

METHODS

Lens epithelial explants and whole lenses from weanling rats were cultured with TGF-beta1, TGF-beta2, or TGF-beta3 at concentrations ranging from 0.025 ng/ml to 4 ng/ml for 3 to 5 days. Cataractous changes were monitored daily by phase contrast microscopy and by immunofluorescent detection of cataract markers alpha-smooth muscle actin and type I collagen. Expression of TGF-beta was studied by immunofluorescence and in situ hybridization on eye sections from neonatal and weanling rats.

RESULTS

All three isoforms induced morphologic changes in lens epithelial explants and cultured lenses that are typically associated with human subcapsular cataract. Transforming growth factor-beta2 and TGF-beta3 were approximately 10 times more potent than TGF-beta1. All three isoforms were expressed in the eye in spatially distinct but overlapping patterns. Transforming growth factor-beta1 and TGF-beta2 and their mRNA were detected in most ocular tissues, including the lens. Although TGF-beta3 was immunolocalized in lens epithelium and fibers and in other ocular tissues, its mRNA was detected only in the retina and choroid.

CONCLUSIONS

All three isoforms of TGF-beta are potentially available to lens cells and have the potential to induce cataractous changes. The results suggest that TGF-beta activity is normally tightly regulated in the eye. Activation of TGF-beta in the lens environment, such as may occur during injury, in wound healing, or in pathologic conditions may contribute to cataractogenesis in vivo.

Differential expression of fibroblast growth factor receptors during rat lens morphogenesis and growth

PURPOSE

Fibroblast growth factors (FGF) play important roles in the developmental biology of the lens. Recently, it was shown that the expression of one of the FGF receptors, FGFR1 (flg; fibroblast growth factor receptor 1), was closely associated with the onset of lens fiber differentiation. In this study, the expression patterns of three other members of the FGF receptor family were analyzed and compared.

METHODS

The expression patterns of FGFR2 (bek and keratinocyte growth factor receptor [KGFR] variants) and FGFR3 were analyzed by in situ hybridization during embryonic and postnatal lens development.

RESULTS

In the ocular primordia, both FGFR2 variants were detected on embryonic day 12 (E12) and FGFR3 was detected on E14. From E16 to E20, distinct spatial expression patterns became evident within the lens; FGFR3 showed an anteroposterior increase in expression, with strongest expression in the outer cortical fibers. In contrast, bek showed uniform expression throughout the lens epithelium (including the central and germinative zones) and the transitional zone, with a subsequent decline in maturing fibers. The KGFR variant of FGFR2 showed strongest expression in the early fibers of the transitional zone; its expression in the epithelium was weaker in the germinative zone of embryonic and neonatal rats. There was an age-related decline in expression of FGFRs after birth-an effect that was more marked for FGFR3 than for the FGFR2 variants.

CONCLUSIONS

Combined with those in a previous study, these results indicate that the FGFR1, bek, KGFR, and FGFR3 genes exhibit different, yet overlapping, patterns of expression throughout lens development and differentiation. The distinct spatiotemporal patterns of expression of FGF receptors may play an important role in regulating anteroposterior patterns of lens cell behavior.

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.

Mutation in Sos1 dominantly enhances a weak allele of the EGFR, demonstrating a requirement for Sos1 in EGFR signaling and development

We have investigated the role of the mammalian Son of sevenless 1 (Sos1) protein in growth factor signaling in vivo by generating mice and cell lines that lacked the Sos1 protein. Homozygous null embryos were smaller than normal, died mid-gestation with cardiovascular and yolk sac defects, and their fibroblasts showed reduced mitogen-activated protein kinase activation in response to epidermal growth factor (EGF). An intercross of mice mutant for Sos1 and the EGF receptor (EGFR) demonstrated that a heterozygous mutation in Sos1 dominantly enhanced the phenotype of a weak allele of the EGFR allele (wa-2). These animals had distinctive eye defects that closely resembled those seen in mice that were null for the EGFR or its ligand, TGF alpha. Our findings provide the first demonstration of a functional requirement for Sos1 in growth factor signaling in vivo. They also show that the genetic test of enhancement of weak receptor allele by heterozygous mutation in one component represents a powerful tool for analyzing the ras pathway in mammals.

Estrogen protects lenses against cataract induced by transforming growth factor-beta (TGFbeta)

Cataract, already a major cause of visual impairment and blindness, is likely to become an increasing problem as the world population ages. In a previous study, we showed that transforming growth factor-beta (TGFP) induces rat lenses in culture to develop opacities and other changes that have many features of human subcapsular cataracts. Here we show that estrogen protects against cataract. Lenses from female rats are more resistant to TGFbeta-induced cataract than those from males. Furthermore, lenses from ovariectomized females show increased sensitivity to the damaging effects of TGFbeta and estrogen replacement in vivo, or exposure to estrogen in vitro, restores resistance. Sex-dependent and estrogen-related differences in susceptibility to cataract formation, consistent with a protective role for estrogen, have been noted in some epidemiological studies. The present study in the rat indicates that estrogen provides protection against cataract by countering the damaging effects of TGFbeP. It also adds to an increasing body of evidence that hormone replacement therapy protects postmenopausal women against various diseases.

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.

IGF enhancement of FGF-induced fibre differentiation and DNA synthesis in lens explants

The aim of this study was to carry but an analysis of the effects of insulin-like growth factors (IGFs) on responses induced by FGF in lens epithelial explants. Central explants from postnatal rats were cultured with concentrations of FGF-2 known to stimulate fibre differentiation or cell proliferation, with and without IGF-I or IGF-II at concentrations ranging from 0-1000 ng ml-1. Fibre-specific beta- and gamma-crystallin accumulation was determined by ELISA after 5-10 days culture and [3H]thymidine incorporation was assessed at 18-24 hr. Generally, both FGF and IGF were added on day 0. In the absence of FGF, IGF induced significant DNA synthesis, but negligible fibre differentiation. When included with FGF, however, IGF synergistically enhanced both DNA synthesis and the accumulation of fibre-specific crystallins. For beta-crystallin, it was shown that this enhancement reflected a substantial increase in the amount of crystallin in individual cells, not merely an increase in cell numbers. The potentiating effects of IGF-I and IGF-II were comparable. For the fibre differentiation response, it was shown that the cells remained responsive to the synergistic influence of IGF-1 for up to 4 days of culture. The dose response characteristics of the fibre differentiation response suggest that mediation occurs mainly through the IGF-I receptor. Because IGF, as well as FGF, is known to be present in the ocular media, IGF may have a role in modulating FGF-induced responses in the lens in vivo.

FGF receptor-1 (flg) expression is correlated with fibre differentiation during rat lens morphogenesis and growth

Our previous studies indicate an important role for fibroblast growth factor (FGF) in lens development. Here we study the expression of the flg variant of FGF receptor 1 (FGFR1) during lens development by immunohistochemistry and in situ hybridisation. FGFR1 was expressed throughout lens development. Prominent FGFR1 immunoreactivity was associated with cell nuclei, particularly in differentiating lens fibres, suggesting internalisation and nuclear translocation of the receptor. FGFR1 immunoreactivity was also associated with basolateral membranes of cells in the equatorial region and at lens sutures. FGFR1 mRNA was only weakly expressed during early lens morphogenesis but expression increased with the onset of lens fibre differentiation. Once the lens acquired its distinct polarity, an anteroposterior gradient in both protein reactivity and mRNA signal was evident. Anteriorly, central epithelial cells showed weak expression for FGFR1, whereas more posteriorly, in the germinative and transitional zones of the lens where cells maximally proliferate and undergo early stages of fibre differentiation, respectively, expression was significantly stronger. The anteroposterior gradient of increased expression of FGFR1 in the lens coincides with the previously documented anteroposterior gradient of FGF stimulation. In lens epithelial explants, FGF stimulation was found to upregulate FGFR1 expression. Such upregulation may be an important mechanism for generating a high level of FGF stimulation and ensuring a fibre differentiation response. In postnatal rat lenses, there was a significant age-related decline in FGFR1 expression; this correlates with the reduced rate of lens fibre differentiation with age. Overall, these studies support the hypothesis that FGF and FGFR1 are important for regulation of lens fibre differentiation throughout lens development.

Lens tropomodulin: developmental expression during differentiation

Lens epithelial cells undergo a dramatic transformation during the process of differentiation into elongated fiber cells. The membrane-associated actin cytoskeleton is likely to play a critical role in the stabilization and maintenance of the highly elongated fiber cell shape. Tropomodulin is a tropomyosin-binding protein associated with actin filaments in a variety of terminally differentiated cell types where stable actin filament organization is required for cell function. We now present results of studies to determine the temporal expression of tropomodulin in the developing lens. In situ hybridization experiments detected expression of tropomodulin mRNA in the developing mouse lens in elongating cells with a pattern similar to that of the fiber specific beta- and gamma-crystallins. Tropomodulin mRNA expression first appeared around 11.5 days post-coitum in elongating cells in the posterior part of the lens vesicle. At later stages the signal for tropomodulin was present in the elongating cells at the lens equator and in cortical fiber cells; signal was absent from the epithelium. To investigate the possible link between tropomodulin expression and fiber differentiation we used a well-established lens epithelial explant culture system in which fiber differentiation is induced by fibroblast growth factor (FGF). Tropomodulin expression was only observed in FGF-treated explants in conjunction with morphologic changes characteristic of lens fiber cell differentiation. The appearance of tropomodulin during the process of fiber cell differentiation suggests that tropomodulin may be important for stabilization and/or determination of actin filament length.

Inhibition of transforming growth factor-beta-induced cataractous changes in lens explants by ocular media and alpha 2-macroglobulin

PURPOSE

To investigate the ocular media for the presence of inhibitors of transforming growth factor-beta (TGF beta) using a lens epithelial explant system in which TGF beta induces cataractous changes. The effect of alpha 2-macroglobulin, an inhibitor of TGF beta in other systems, also was assessed.

METHODS

Explants prepared from 21-day-old rats were cultured with TGF beta 2 with and without 50% bovine aqueous or vitreous or alpha 2-macroglobulin. alpha 2-macroglobulin was added to an aqueous concentrate, shown to contain endogenous TGF beta activity by blocking with anti-TGF beta. Explants were monitored by phase-contrast microscopy for 5 days and assessed in terms of capsule wrinkling, spindle-cell formation, blebbing, and cell loss. alpha 2-macroglobulin in the ocular media was assessed by enzyme-linked immunosorbent assay and Western blot analysis.

RESULTS

At 50% strength, neither aqueous nor vitreous demonstrated TGF beta-like activity; however, aqueous partially and vitreous completely prevented cataractous changes induced by 25 and 100 pg/ml TGF beta 2, respectively. alpha 2-macroglobulin (50 to 200 micrograms/ml) also protected against these changes, with complete inhibition of TGF beta 2 or aqueous-derived TGF beta activity at the highest concentration. A threefold higher concentration of alpha 2-macroglobulin was detected in vitreous than aqueous.

CONCLUSIONS

Both aqueous and vitreous contain molecule(s) that inhibit TGF beta 2 activity. alpha 2-macroglobulin has been identified in the ocular media and shown to block cataractous changes induced by TGF beta. Maintaining appropriate levels of alpha 2-macroglobulin or similar molecules in the ocular media may protect lens cells from the damaging effects of TGF beta, and reduced levels may predispose to cataract.

Cataract induction in lenses cultured with transforming growth factor-beta

PURPOSE

Anterior subcapsular cataracts are characterized by the appearance of opaque plaques of abnormal cells. Distinctive spindle-shaped cells containing alpha-smooth muscle actin are present and are associated with wrinkling of the overlying lens capsule. Accumulations of extracellular matrix, including type I collagen, also are found. The authors previously reported that transforming growth factor-beta (TGF-beta) induces similar aberrant morphologic changes in lens epithelial explants. More recently, they identified alpha-smooth muscle actin in explants cultured with TGF-beta. The aim of this study was to determine whether TGF-beta induces comparable cataractous changes in whole lenses and to examine the effects of this treatment on the transparency of the lens.

METHODS

Whole lenses from 21-day-old rats were cultured in defined serum-free medium with TGF-beta 2 or without added growth factors for 5 days. Lenses were then photographed and prepared for histology and immunolocalization.

RESULTS

Lenses cultured with TGF-beta developed distinct anterior opacities just beneath the lens capsule. Histologically, clumps of abnormal cells corresponded with these opacities. Spindle-shaped cells, which contained alpha-smooth muscle actin, were present, and the overlying capsule was often wrinkled. The clumps contained accumulations of type I collagen, laminin, and heparan sulphate proteoglycan. In contrast, lenses cultured without growth factors remained transparent, retained normal lens morphology, and did not accumulate alpha-smooth muscle actin or type I collagen.

CONCLUSIONS

These results show that TGF-beta induces whole lenses to form opacities that contain morphologic and biochemical markers for subcapsular cataract.

Differential effects of aqueous and vitreous on fiber differentiation and extracellular matrix accumulation in lens epithelial explants

PURPOSE

Results from this and other laboratories strongly suggest that differences in the properties of the ocular media that bathe cells in the anterior and posterior regions of the lens contribute to its normal growth patterns and polarity. The aim of this study was to compare the effects of aqueous and vitreous on the morphology of lens epithelial explants, with particular attention to changes associated with fiber differentiation.

METHODS

Light and electron microscopy were used to assess rat lens epithelial explants cultured with bovine aqueous or vitreous. Immunohistochemistry was used to detect fiber-specific crystallins and extracellular matrix components, and synthesis of extracellular matrix was investigated by autoradiography.

RESULTS

Vitreous, but not aqueous, induced morphologic changes characteristic of fiber differentiation, which included cell elongation, organelle loss, and the appearance of ball and socket junctions, as well as the accumulation of beta-crystallin. In addition, vitreous stimulated the synthesis and organization of a distinct basement membrane on explants that resembled the lens capsule, both structurally (regular layers of basal laminae) and immunologically (reactive for laminin and heparan sulphate proteoglycan).

CONCLUSIONS

Only one of the ocular media, the posteriorly located vitreous, induced lens epithelial explants to undergo morphologic events characteristic of fiber differentiation. This provides further support for the hypothesis that anteroposterior patterns of cellular responses in the lens are caused by differences in the ocular media. The observation that vitreous also stimulated the synthesis and assembly of capsule-like extracellular matrix suggests that vitreous contains factors that may influence lens capsule formation in situ.

TGF-beta 1 induces lens cells to accumulate alpha-smooth muscle actin, a marker for subcapsular cataracts

Spindle-shaped myofibroblast-like cells, which contain alpha-smooth muscle actin, have been described in anterior subcapsular cataract and after-cataract. In a previous study in this laboratory, it was shown that transforming growth factor-beta (TGF beta) induces the formation of spindle-shaped cells in lens epithelial explants. The aim of this investigation was to determine whether these TGF beta-induced spindle-shaped cells contain alpha-smooth muscle actin. Lens epithelial explants were prepared from 21-day-old rats and cultured with either TGF beta 1 or basic FGF alone, a combination of both growth factors, or without added growth factors. After three days, cellular changes were monitored by phase contrast microscopy, localisation of filamentous actin with rhodamine-phalloidin, and immunolocalisation and immunoblotting of alpha-smooth muscle actin. TGF beta induced rapid cell elongation and formation of characteristic spindle-shaped cells in lens epithelial explants in the presence or absence of FGF. These cells contained alpha-smooth muscle actin, a marker for myofibroblastic cells and a protein not normally found in the lens. The present study thus provides molecular evidence that TGF beta induces cataractous changes in lens epithelial cells. As TGF beta is potentially available to lens cells in situ throughout life, these findings are consistent with a key role for TGF beta in the aetiology of major forms of subcapsular cataract.

Induction of cataract-like changes in rat lens epithelial explants by transforming growth factor beta

PURPOSE

To investigate the possible role of transforming growth factor beta (TGF beta) in lens development and growth, the authors studied the influence of TGF beta, alone and in combination with fibroblast growth factor (FGF), on lens epithelial explants.

METHODS

Lens explants were prepared from both postnatal and adult rats, and changes during 5 days of culture with growth factor(s) were monitored by light and electron microscopy, immunolocalization of laminin, heparan sulfate proteoglycan and fiber-specific crystallins, and crystallin enzyme-linked immunosorbent assays.

RESULTS

TGF beta induced cells in explants to undergo an extensive and rapid elongation with features that distinguished it from FGF-induced fiber differentiation. TGF beta also induced accumulation of extracellular matrix, capsule wrinkling, cell death by apoptosis, and distinctive arrangements of cells. Standard explants from 10-day-old rats responded to TGF beta only in the presence of FGF. Comparable explants from adult rats or from 21-day-old rats (cultured on a laminin substratum) responded readily to TGF beta whether or not FGF was present.

CONCLUSIONS

First, these results suggest a role for TGF beta in regulating normal processes in lens cells such as the production of extracellular matrix and capsule formation. Second, because many of the changes induced by TGF beta resembled changes reported to occur during the formation of various kinds of subcapsular cataracts, the results suggest that detailed studies of factors that influence the ability of lens cells to respond to TGF beta and the bioavailability of TGF beta in the ocular media may provide important insights into the etiology of some forms of cataract.

IGF-1 enhancement of FGF-induced lens fiber differentiation in rats of different ages

PURPOSE

Previously, using lens epithelial explants from neonatal rats, we showed that both insulin and IGF-1 synergistically enhance the effectiveness of FGF as an inducer of fiber differentiation. The authors aimed to determine whether IGF-1 enhances FGF-induced fiber differentiation in lens epithelial cells at various ages and, in particular, whether it can counter a marked age-related decline in responsiveness to FGF noted previously.

METHODS

The effects of IGF-1 and bFGF were assessed using lens epithelial explants from neonatal, weanling, and adult rats. Fiber differentiation (after 13 days' culture) was monitored by crystallin ELISAs of explant lysates and also by immunofluorescent localization of crystallins.

RESULTS

IGF alone had minimal effects. For younger rats, FGF alone enhanced the accumulation of alpha-, beta- and gamma-crystallins throughout explants, the peripheral region being more responsive than the central region. For adult rats, only the peripheral region responded; small amounts of alpha- and beta-crystallins were detected, but gamma-crystallin was not. Combining IGF with FGF induced gamma-crystallin in explants from adult rats (peripheral region) and enhanced the accumulation of all crystallins more than additively at all ages, mainly in the central region (young rats) or only in the peripheral region (adults). Including IGF with FGF prevented an age-related decline in the beta/alpha-crystallin ratio but not in the gamma/beta-crystallin ratio.

CONCLUSIONS

IGF-1 enhances the bFGF-induced fiber differentiation responses of lens epithelial cells in neonatal, weanling, and adult rats, partially restoring an age-related decline in the responsiveness of lens cells to FGF.

Spatio-temporal distribution of acidic and basic FGF indicates a role for FGF in rat lens morphogenesis

As part of an investigation into the role of FGF in lens development, we have studied the distribution of both aFGF and bFGF during eye morphogenesis from embryonic days 10 to 18 (E10-E18) in the rat. For aFGF, reactivity was found only in ectoderm at E10, prior to contact between the optic vesicle and presumptive lens ectoderm. During lens placode formation (E11) there was a transient, diffuse reactivity for aFGF in anterior optic vesicle cells directly apposed to the labelled ectoderm of the lens placode. At E12 the diffuse reactivity of the lens placode had changed to a discrete localisation along the basolateral surfaces of differentiating cells in the lens pit. Similar reactivity was associated with neuroblasts along the inner margin of the optic cup. At the early lens vesicle stage (E13) the baso-lateral aFGF-like reactivity associated with elongating lens cells was more intense and extensive. From the late lens vesicle stage (E14) to E18, reactivity in the lens was increasingly restricted to the equatorial regions which incorporate the germinative and transitional zones. From E16 to E18, aFGF-like reactivity in the retina was predominantly localised in the peripheral regions corresponding to the developing ciliary body and iris and in the central retina associated with ganglion cell axons. For bFGF, weak reactivity was detectable as early as E13 in the developing lens capsule and increased in intensity during lens development with the posterior capsule reacting more intensely than the anterior capsule. Retinal bFGF-like reactivity was first detected at E14, associated with differentiating ganglion cells in the central retina. From E16 to E18 the retinal ganglion cells showed increasing reactivity and the pattern of reactivity followed the centro-peripheral pattern of retinal development. Thus reactivity for aFGF is first detected in presumptive lens ectoderm and subsequently in optic vesicle cells which are closely associated with lens ectoderm. This raises the possibility that aFGF may be involved in inductive interactions between presumptive lens ectoderm and optic vesicle. Furthermore the localisation patterns established for both aFGF and bFGF during lens and retina morphogenesis suggest an important role for FGF in regulating their morphogenesis and growth.

Localization of acidic fibroblast growth factor, basic fibroblast growth factor, and heparan sulphate proteoglycan in rat lens: implications for lens polarity and growth patterns

PURPOSE

Previous research in this laboratory has shown that fibroblast growth factor stimulates lens epithelial explants to proliferate, migrate, and differentiate into fibers in a progressive dose-dependent manner. The lens has distinct compartments where cells proliferate (germinative zone), migrate, or get displaced (equator) and differentiate into fibers (transitional zone). These compartments occur in an anteroposterior spatial sequence and the authors hypothesized that fibroblast growth factor plays a critical role in determining these spatial patterns of lens growth and lens polarity. To investigate this hypothesis the distribution of fibroblast growth factor in the lens was analyzed.

METHODS

Immunohistochemistry was used to localize acidic fibroblast growth factor and basic fibroblast growth factor in the cells and capsule of lenses from neonatal, weanling, and adult rats. Because of its functional relationship with fibroblast growth factor, heparan sulphate proteoglycan was also localized in the lens.

RESULTS

In all ages examined, cytoplasmic acidic fibroblast growth factor is present in the germinative and transitional zones of the lens and both acidic fibroblast growth factor and basic fibroblast growth factor are present in the capsule. A major finding is the co-localization of fibroblast growth factor and heparan sulphate proteoglycan reactivity in the lens capsule in the form of laminae. These laminae become more prominent as the capsule thickens and differences in arrangement of laminae between anterior, equatorial, and posterior regions of the capsule also become apparent.

CONCLUSIONS

The presence of fibroblast growth factor in lens cells and capsule in neonatal, weanling, and adult rats indicates an important role for fibroblast growth factor in lens cell biology. Moreover, the regional distribution of fibroblast growth factor, particularly in the lens cells, indicates that it may influence determination of lens polarity and growth patterns.

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

We have shown previously that FGF induces lens epithelial cells in explant culture to proliferate, migrate and differentiate into fibre cells in a progressive concentration-dependent manner. In situ, these processes occur in a distinct anterior-posterior pattern in clearly defined regions of the lens. Thus anterior-posterior differences in the bio-availability of FGF in the lens environment may play a role in determining lens polarity and growth patterns. In this study, using heparin chromatography and western blotting (or ELISA), we established that both acidic and basic FGF are present in the aqueous and vitreous (the ocular media that bathe the anterior and posterior compartments of the lens, respectively). In addition, substantially more FGF was recovered from vitreous than from aqueous. Both forms of FGF were also detected in lens fibre cells and capsule. A truncated form of basic FGF (less than 20 × 10(3) M(r)) predominated in every case with traces of higher M(r) forms in lens cells. For acidic FGF, the classical full-length form (about 20 × 10(3) M(r)) predominated in lens cells and a truncated form was found in vitreous. The capsule contained a higher M(r) form. Using our explant system, we also tested the biological activity of ocular media and FGF fractions obtained from vitreous and lens cells. Vitreous but not aqueous contained fibre-differentiating activity. Furthermore, virtually all the fibre-differentiating activity of vitreous was shown to be FGF-associated, as follows: (a) this activity remained associated with FGF during fractionation of vitreous by heparin and Mono-S chromatography and (b) the activity of the major FGF-containing fraction was blocked by antibodies to acidic and basic FGF. Posterior, but not anterior, capsule was shown to have mitogenic activity, which was neutralised by FGF antibodies and associated only with the cellular surface. These results support our hypothesis that FGF is involved in determining the behaviour of lens cells in situ. In particular, a key role for FGF in determining lens polarity and growth patterns is suggested by the anterior-posterior differences in the bio-availability of FGF in the ocular media and capsule.

Age of rats affects response of lens epithelial explants to fibroblast growth factor

Fibroblast growth factor (FGF) is a potent inducer of fibre differentiation in lens epithelial explants from neonatal rats. Previously, using explants prepared from the central region of the lens epithelium, we showed an age-related loss of ability to accumulate fibre-specific crystallins in response to basic FGF. These studies have now been extended to include the peripheral region of the lens epithelium. Firstly we cultured explants from the central or peripheral regions of neonatal lenses with varying doses of FGF for 5 days, then determined how much fibre-specific beta-crystallin they had accumulated. The concentration of FGF required to induce a half-maximal response was lower for peripheral than for central cells (7 ng ml-1 compared with 36 ng ml-1). We then compared the ability of peripheral explants from 3-, 21-, 100- and 175-day-old rats to undergo fibre differentiation during culture with FGF for 13 days. In these studies alpha-, beta- and gamma-crystallins were localized in explants or quantified by ELISAs. There was an age-related decrease in responsiveness to FGF, as already observed for central explants; however, unlike central explants, peripheral explants from the oldest rats still retained the ability to respond to FGF by accumulating beta-crystallin. This suggests that FGF in the eye may play an important role in inducing lens epithelial cells at the lens equator to differentiate into fibres throughout life.

Rise and fall of crystallin gene messenger levels during fibroblast growth factor induced terminal differentiation of lens cells

Explanted rat lens epithelial cells differentiate synchronously in vitro to lens fiber cells in the presence of basic fibroblast growth factor (bFGF). We have monitored the expression of the three rat crystallin gene families, the alpha-, beta-, and gamma-crystallin genes, during this process. The expression of these gene families is sequentially activated, first the alpha-crystallin genes at Day 1, then the beta-crystallin genes at Day 3, and finally the gamma-crystallin genes at Day 8. The steady state levels of alpha- and beta-crystallin mRNA are not affected by incubation with actinomycin D, suggesting that these mRNAs are stable. Nevertheless, all crystallin mRNAs disappear from the differentiated explants between Days 10 and 11, a process signaled by bFGF. At this time a novel abundant mRNA appears. Cloning and sequencing showed that this mRNA encoded aldose reductase. Our results suggest a novel model for the regulation of crystallin synthesis during lens cell differentiation: a gene pulse delivers a certain amount of stable mRNA, this mRNA is removed at a later stage of differentiation by a stage-specific breakdown mechanism. Each of these regulatory steps requires a signal from bFGF.

The age of rats affects the response of lens epithelial explants to fibroblast growth factor. An ultrastructural analysis

Fibroblast growth factor (FGF) is a potent inducer of fiber differentiation in lens epithelial explants from neonatal rats as assessed by the accumulation of fiber-specific proteins (beta- and gamma-crystallins) and the progression of cells through a sequence of morphologic events characteristic of fiber differentiation in situ. Because new fibers normally are formed in the lens throughout life, the authors questioned whether epithelial cells from rats of all ages are induced to differentiate into fibers by FGF. Earlier studies have shown that, with the increasing age of the donor rat, the lens epithelial explants had a reduced ability to accumulate beta- and gamma-crystallins in response to FGF. To determine if the characteristic morphologic events in fiber differentiation were induced by FGF in explants from rats of different ages, an ultrastructural study was done. Using the time of appearance and level of expression of the following morphologic markers of fiber differentiation: (1) cell elongation, (2) reduction of cytoplasmic organelles, (3) formation of cell processes, and (4) fiber denucleation, the level of fiber differentiation induced by FGF was assessed in explants from 10-, 21-, 100-, and 175-day-old rats. These results showed that, with increasing donor age, epithelial cells showed a gradual decline in responsiveness to FGF. This was manifested by a slower progression through the sequence of fiber-specific structural events as the age of the donor rat increased. At all ages studied, cells in the central region of explants responded more slowly than cells from the peripheral region. The finding that FGF induces events in fiber differentiation, albeit at a slower rate, in explants from mature rats supports the hypothesis that FGF in the eye continues to play a role in inducing lens epithelial cells at the lens equator to differentiate into fibers throughout life.

Distribution of acidic and basic fibroblast growth factors (FGF) in the foetal rat eye: implications for lens development

Previously we reported that, in vitro, lens cells proliferate, migrate or differentiate in response to low, medium and high concentrations of FGF respectively. To examine further the role of FGF in lens development we used immunohistochemistry to study the distribution of aFGF and bFGF in the eye of the 20 day rat foetus. Strong aFGF-like reactivity was localised in a band of cells near the lens equator which included the germinative zone where most cell proliferation occurs and the transitional zone where epithelial cells differentiate into fibres. The closely apposed inner epithelial layer of the ciliary and iridial retina also reacted strongly. Reactivity for aFGF was also found in the epidermis and in the corneal and conjunctival epithelia. In the neural retina, reactivity was found in the nerve fibre layer and in isolated cells of the inner plexiform layer. bFGF-like reactivity was found in the retinal ganglion cell layer, extra-ocular muscles and associated with endothelial cells of the hyaloid, lenticular and choroid vasculatures. Pre-digestion of sections with hyaluronidase caused loss of cell-associated reactivity but revealed strong bFGF-like reactivity in ocular basement membranes, in particular, the lens capsule. The sensitivity of this capsular bFGF localisation to heparinase indicates that bFGF in the extracellular matrix is complexed with heparan sulphate proteoglycans. The results of this study are consistent with the hypothesis that FGF plays an important role in lens development via both autocrine and paracrine mechanisms.

The role of fibroblast growth factor in eye lens development

In this review we have presented evidence that FGF plays an important role in inducing events in lens morphogenesis and growth. Our studies show that FGF stimulates lens epithelial cells in explants to proliferate, migrate, and differentiate into fibers at low, medium, and high concentrations, respectively. This has some important implications for understanding the behavior of lens cells in the eye. The fact that aFGF is detected in the equatorial region of the lens where cells are actively proliferating, possibly migrating, and differentiating into fibers suggests that these events may be under autocrine control in vivo, at least to some extent. Because FGF is also present in the ciliary and iridial region of retina and in the vitreous, paracrine control may also be involved. Cell proliferation, fiber differentiation, and (possibly) cell migration occur in characteristic spatial patterns that are related to distinct compartments of the lens. We suggest that cells in the germinative zone receive only a low level of FGF stimulation arising from the cells themselves and possibly also from the ciliary and iridial regions of the retina but, whatever the source, this is only sufficient to stimulate proliferation. Lens epithelial cells that migrate or are displaced into the transitional zone below the lens equator receive some FGF from these sources but in addition receive a strong stimulus from the high level of FGF in the vitreous; thus, fiber differentiation is induced. Cells at the junction between these two zones may receive an intermediate level of FGF stimulation, sufficient to induce cell migration. In essence, we are proposing that, in the eye, FGF acts as a lens morphogen in the sense that different levels of FGF stimulation elicit different lens cell responses. Hence its characteristic distribution in the eye establishes lens polarity and growth patterns. Since FGF has an inductive effect on lens cells from mature age animals, we also propose that this specific distribution of FGF in the eye is also important for maintenance of a normal lens throughout life. Finally the synergistic effects of insulin/IGF on the FGF-induced responses highlight the importance of considering the distribution of members of the insulin/IGF family of molecules in vivo. Mechanisms that control levels of both the FGF and insulin/IGF families of factors in the eye are probably of crucial importance in the formation and maintenance of a normal lens.

The roles of laminin and fibronectin in the development of the lens capsule

This study examines the distribution of laminin and fibronectin in the rat lens capsule during development. Both these extracellular matrix glycoproteins are localised in the interspace between presumptive lens and presumptive retina as well as in their basal laminae. The lens capsule arises from multilayering of the basal lamina of the lens cells. Immunofluorescence localises both laminin and fibronectin in the capsule at 16 days of embryonic development, although reactivity for fibronectin is much weaker than for laminin. In the 19 day embryo only laminin is detected. This indicates that during embryonic development fibronectin becomes a minor component of lens cell ECM and is not accumulated in the developing capsule. The roles of laminin and fibronectin in promoting cell migration during development were analysed in explant cultures. Lens epithelial explants from 16, 17 and 19 day old embryos and neonatal rats were grown on a laminin or fibronectin substratum. Lens cells from all ages of rats migrated on the laminin substratum, whereas lens cells progressively lost the ability to migrate on a fibronectin substratum as the age of the donor increased. This developmental loss of ability to migrate on fibronectin in vitro coincides with the developmental loss of fibronectin from the lens capsule in vivo. Therefore, we propose that whilst both laminin and fibronectin may be important for promoting migration of lens cells on their substratum at early stages of lens morphogenesis, during development laminin takes over as the key molecule that promotes migration on the capsule.