Lens differentiation in vertebrates. A review of cellular and molecular features

Zebrafish mutagenesis yields eye morphological mutants with retinal and lens defects

A chemical mutagenesis to identify zebrafish eye morphological mutants was performed by screening F(3) larvae at 5 and 7 days post-fertilization (dpf) for changes in eye or pupil size. Based on histological analysis, four different phenotypic classes were obtained. The two Class I and three Class II mutants are all characterized by small eyes and exhibit defects in early retinal development or unregulated cell death, respectively. The single Class III mutant has reduced ocular pigmentation. The three Class IV mutants display defects in the ocular lens, including one mutant line with normal sized eyes and pupils that develops lens opacity at 7 dpf.

Rho GTPase inactivation impairs lens growth and integrity

To elucidate the significance of Rho GTPase signaling on lens growth and structural integrity, we have selectively inactivated Rho GTPase in the ocular lens. To achieve this tissue-specific inactivation, a transgene encoding the C3-exoenzyme from Clostridium botulinum has been expressed in mice under transcriptional control of the lens-specific alphaA-crystallin promoter. C3-exoenzyme is known to selectively inactivate all Rho GTPase isoforms by ADP-ribosylating an asparagine residue at position 41. Mice expressing the C3-exoenzyme transgene exhibited selective ocular defects, including cataract and microphthalmia. Extralenticular effects included ocular hemorrhage (blood accumulation in the anterior and posterior chambers of the eye) and abnormalities of the iris including focal attachments to lens and cornea (synechiae). C3-transgene expression was found only in the lens and not in the other ocular tissues as determined by RT-PCR analysis. Histologic examination of the eyes of C3 transgenic mice from two independent lines revealed extensive abnormalities of the lens, including defective fiber cell differentiation and elongation, ruptured posterior lens capsule, and thickened anterior lens capsule. Electron microscopic analysis of hemorrhaged C3 eyes showed abnormalities in the posterior hyaloid vessels. Collectively these data reveal the importance of Rho GTPase signaling in regulating lens growth and maintenance of lens transparency.

Unique and redundant connexin contributions to lens development

Connexin genes encode intercellular channels that help to coordinate development. In mice, the targeted deletion of different connexins produces disparate effects on ocular growth and differentiation in the lens, and the need for multiple channel subunits is poorly understood. Knockout of Cx46 causes a loss of homeostasis and cataracts. Deletion of Cx50 results in reduced ocular growth and cataracts. Targeted replacement of Cx50 with Cx46 by genetic knock-in corrected defects in cellular differentiation and prevented cataracts, but did not restore normal growth. These data show that intrinsic properties of Cx50 were required for cellular growth, whereas nonspecific restoration of communication by Cx46 maintained differentiation.

Disruption ofGja8(α8 connexin) in mice leads to microphthalmia associated with retardation of lens growth and lens fiber maturation

The development of the vertebrate lens utilizes a sophisticated cell-cell communication network via gap junction channels, which are made up of at least three connexin isoforms, α8 (Cx50), α3 (Cx46) and α1 (Cx43), and which are encoded by three different genes. In a previous study, we reported that, with a disruption of Gja3 (α3 connexin), mice developed nuclear cataracts with a normal sized lens. We show that Gja8tm1 (α8–/–) mice develop microphthalmia with small lenses and nuclear cataracts, while the α8 heterozygous (+/–) mice have relatively normal eyes and lenses. A comparative study of these α3 and α8 knockout mice showed that the protein levels of both α3 and α8 were independently regulated and there was no compensation for either the α3 or α8 protein from the wild-type allele when the other allele was disrupted. More interestingly, western blotting data indicated that the presence of α8 in the lens nucleus is dependent on α3 connexin, but not vice versa. The staining of the knock-in lacZ reporter gene showed the promoter activity of α8 connexin is much higher than that of α3 connexin in embryonic lenses and in adult lens epithelium. More importantly, a delayed denucleation process was observed in the interior fibers of the α8–/– lenses. Therefore, α8 connexin is required for proper fiber cell maturation and control of lens size.

Optical dysfunction of the crystalline lens in aquaporin-0-deficient mice

Aquaporin-0 (AQP0), a water transport channel protein, is the major intrinsic protein (MIP) of lens fiber cell plasma membranes. Mice deficient in the gene for AQP0 (Aqp0, Mip) were generated from a library of gene trap embryo stem cells. Sequence analysis showed that the gene trap vector had inserted into the first exon of Aqp0, causing a null mutation as verified by RNA blotting and immunochemistry. At 3 wk of age (postnatal day 21), lenses from null mice (Aqp0(-/-)) contained polymorphic opacities, whereas lenses from heterozygous mice (Aqp0(+/-)) were transparent and did not develop frank opacities until approximately 24 wk of age. Osmotic water permeability values for Aqp0(+/-) and Aqp0(-/-) lenses were reduced to approximately 46% and approximately 20% of wild-type values, respectively, and the focusing power of Aqp0(+/-) lenses was significantly lower than that of wild type. These findings show that heterozygous loss of AQP0 is sufficient to trigger cataractogenesis in mice and suggest that this MIP is required for optimal focusing of the crystalline lens.

Arrested differentiation and epithelial cell degeneration in zebrafish lens mutants

In a chemical mutagenesis screen, we identified two zebrafish mutants that possessed small pupils. Genetic complementation revealed these two lines are due to mutations in different genes. The phenotypes of the two mutants were characterized using histologic, immunohistochemical, and tissue transplantation techniques. The arrested lens (arl) mutant exhibits a small eye and pupil phenotype at 48 hr postfertilization (hpf) and lacks any histologically identifiable lens structures by 5 days postfertilization (dpf). In contrast, the disrupted lens (dsl) mutants are phenotypically normal until 5 dpf, and then undergo lens disorganization and cell degeneration that is apparent by 7 dpf. Histology reveals the arl mutant terminates lens cell differentiation by 48 hpf, whereas the dsl lens exhibits a defective lens epithelial cell population at 5 dpf. Lens transplantation experiments demonstrate both mutations are autonomous to the lens tissue. Immunohistochemistry reveals the retinal cells may suffer subtle effects, possibly due to the lens abnormalities.

Functional analysis of the promoter and chromosomal localization for human LEP503, a novel lens epithelium gene

LEP503 is a novel gene product isolated from lens epithelial cells by a subtractive cDNA cloning strategy. It is highly conserved in different vertebrate species and developmentally regulated in postnatal rat lens, suggesting that LEP503 may be an important lens epithelium gene involved in the processes of lens epithelial cell differentiation. The expression of LEP503 is highly restricted to lens epithelial cells in vivo. To investigate the molecular mechanisms regulating the promoter of the human LEP503, we cloned and characterized the promoter of the human LEP503 gene. The transcription start site was localized to a nucleotide C 22 base pairs (bp) 5' of the initiation methionine codon. By reporter gene transfection experiments, we found that approximately 2.5-kb of LEP503 5'-flanking sequence directed high level luciferase activity in human lens epithelial cells; further deletion analysis revealed positive regulatory element between bp -401 and +22. Mutation analysis in each of the seven potential binding sites for transcription factors within the region between -401 and +22 showed that the AP-1 element at -131 and the Sp1 element at -48 are the most important sites for the tissue-specific expression of LEP503. Consistent with lens epithelial cell-restricted expression of LEP503 mRNA, we found that the approximately 2.5-kb 5'-flanking sequence directed high-level promoter activity in lens epithelial cells but not in other cell types. Understanding the LEP503 promoter will allow us to investigate lens epithelial cell-specific gene regulation and to uncover methods for targeting gene delivery specifically to lens epithelial cells. The LEP503 gene is mapped to human chromosome 1q22, the same location to which zonular pulverulent cataract was previously mapped.

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.

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.

Dosage requirement and allelic expression of PAX6 during lens placode formation

Pax6 is a member of the mammalian Pax transcription factor family. Many of the Pax genes display semi-dominant loss-of-function heterozygous phenotypes, yet the underlying cause for this dosage requirement is not known. Mice heterozygous for Pax6 mutations exhibit small eyes (Sey) and in embryos the most obvious defect is a small lens. We have studied lens development in Pax6(Sey)(−1Neu)/+ embryos to understand the basis of the haploinsufficiency. The formation of the lens pre-placode appears to be unaffected in heterozygotes, as deduced from the number of cells, the mitotic index, the amount of apoptosis and the expression of SOX2 and Pax6 in the pre-placode. However, the formation of the lens placode is delayed. The cells at the edge of the lens cup fail to express N-cadherin and undergo apoptosis and the lens fails to detach completely from the surface ectoderm. After formation, the lens, which has 50% of the cells found in wild-type embryos, grows at a rate that is indistinguishable from wild type. We rule out the possibility that monoallelic expression of Pax6 at the time of lens placode formation accounts for the 50% reduction in cell number by showing that expression of Pax6 is biallelic in the lens placode and optic vesicle. We propose instead that a critical threshold of PAX6 protein is required for lens placode formation and that the time in development at which this level is reached is delayed in heterozygotes.

Sequential activation of transcription factors in lens induction

Since the pioneering work of the early 1900s, the lens has been used as a model system for the study of tissue development in vertebrates. A number of embryological transplantation experiments designed to elucidate the role of tissue interactions in the formation of the lens have led to the proposal of a stepwise determination model. This model has recently been refined through the identification of certain transcription factor genes, which exhibit distinct expression patterns and functional properties in the lens cell lineage. Otx2, Pax6, and Lens1 are induced by the adjacent anterior neural plate and expressed in predifferentiated lens ectoderm. Contact between the optic vesicle and lens ectoderm promotes expression of mafs, Soxs, and Prox1, which are responsible for the initiation of lens differentiation programs including crystallin expression, cell elongation, and cell cycle arrest. Further analysis of the expression and functional characteristics of these transcription factors will allow greater detail when describing the orchestration of genetic programs, which control tissue development from induction to maturation.

Inhibition of cell death by lens-specific overexpression of bcl-2 in transgenic mice

Previous studies on cell cycle regulation in the ocular lens using transgenic mice have shown that inactivation of the retinoblastoma tumor suppressor protein (pRb) can cause postmitotic lens fiber cells to enter the cell cycle. However, when the p53 gene and protein are intact, inactivation of pRb in this terminally differentiated cell type results in cell death, rather than continued proliferation. Since bcl-2 has been shown to act as a cell death repressor, the ability of this gene to block p53-dependent apoptosis in lenses was examined. Transgenic mice were generated that overexpress bcl-2 in a lens-specific fashion. Surprisingly, overexpression of bcl-2 was sufficient to interfere with normal fiber cell differentiation, inducing cataracts, microphakia, vacuolization, fiber cell disorganization, and inhibition of fiber cell denucleation. The bcl-2 mice were mated to mice exhibiting lens-specific expression of the N-terminal region of simian virus 40 large T antigen (termed truncT). The resulting double transgenic mice showed a marked reduction in the truncT-induced fiber cell death. Apoptosis in the truncT mice could also be suppressed by crossing these mice into a p53-deficient background. Either overexpression of bcl-2 or loss of p53 in truncT mice resulted in proliferation of fiber cells around the cortex of the lens. These proliferating fiber cells continue to express beta- and gamma-crystallin proteins, which are normally only expressed following withdrawal from the cell cycle. The p53 protein is known to upregulate expression of certain target genes, including p21, a protein that can block cell cycle progression by inhibition of cyclin-dependent kinases. In order to assess whether bcl-2 interferes with the transcriptional activation activity of p53, transgenic lenses were assayed by in situ hybridization for levels of p21 expression. Lenses that expressed both truncT and bcl-2 showed elevated p21, implying that bcl-2 does not inhibit apoptosis by directly inhibiting p53, but instead may block a later step in the apoptosis pathway. In addition, overexpression of p21 is not sufficient to cause apoptosis. These experiments show that the lenses of transgenic mice represent a valuable in vivo setting for studies of both induction and inhibition of programmed cell death.

Forkhead Foxe3 maps to the dysgenetic lens locus and is critical in lens development and differentiation

Here we report the isolation of a novel forkhead gene, Foxe3, that plays an important role in lens formation. During development Foxe3 is expressed in all undifferentiated lens tissues, and is turned off upon fiber cell differentiation. Foxe3 maps to a chromosomal region containing the dysgenetic lens (dyl) mutation. Mice homozygous for dyl display several defects in lens development. dyl mice also show altered patterns of crystallin expression suggesting a dysregulation of lens differentiation. We have identified mutations in Foxe3 that cosegregate with the dyl phenotype and are a likely cause of the mutant phenotype. Head ectoderm expression of Foxe3 is absent in Rx-/- and Small eye embryos indicating that Rx and Pax6 activity are necessary for Foxe3 expression.

Microphthalmia due to p53-mediated apoptosis of anterior lens epithelial cells in mice lacking the CREB-2 transcription factor

CREB-2 (also called ATF4, TAXREB67, or C/ATF) is an evolutionarily conserved member of the CREB/ATF family of basic-leucine zipper transcription factors. CREB-2 is expressed ubiquitously in the adult mouse and can function as both a transcriptional activator and a repressor. However, little was understood about the normal function of CREB-2 in mammalian development or organ physiology. In this report we have used gene targeting to produce CREB-2-deficient (CREB-2-/-) mice. Adult CREB-2-/- mice displayed microphthalmia due to the complete absence of a lens. Early embryonic lens development including formation of the optic vesicle, primary lens fibers, and proliferating anterior epithelial cells occurred normally in these mice. However, beginning at ED 14.5 the CREB-2-deficient anterior epithelial lens cells underwent massive and synchronous apoptosis. This was followed by the complete resorption of the developing lens. Consistent with this defect in anterior epithelial cell survival, in situ hybridization studies showed that CREB-2 is expressed at high levels in wild-type anterior epithelial lens cells at ED 14.5. The defect in lens formation seen in the CREB-2-/- mice was not associated with qualitative defects in the expression of Pax-6, alphaA-crystallin, c-maf, or PDGF-R alpha. However, apoptosis of the anterior epithelial cells was mediated by a p53-dependent cell death pathway because ablation of the p53 gene rescued anterior epithelial cell death and allowed the formation of a lens in the absence of CREB-2. Taken together, these results identify CREB-2 as an important regulator of mammalian lens development.

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

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

N-cadherin function is required for differentiation-dependent cytoskeletal reorganization in lens cells in vitro

Members of the cadherin family of cell adhesion molecules participate in calcium-dependent cell-cell adhesions that are necessary for the cell sorting events that regulate early developmental processes. Although individual cadherin molecules have been shown to participate in tissue histogenesis, the regulation of function of these receptors in cell differentiation has been more difficult to identify. We have determined that N-cadherin linkage to the cytoskeleton is correlated with lens cell differentiation in vivo. Through the use of a chick embryo lens culture system that mimics differentiation in vivo, we have determined that N-cadherin linkage to the cytoskeleton is altered and lens differentiation is blocked by function-blocking antibodies to N-cadherin. In the presence of the N-cadherin function-blocking antibody, NCD-2, both N-cadherin and filamentous actin are prevented from organizing at the cortical membranes. This correlates with an inhibition of lens morphogenesis and differentiation. These results are paralleled by changes in the expression of the molecular components of the cadherin-catenin complex and their linkage to the actin cytoskeleton. In the presence of NCD-2, expression of N-cadherin, alpha-catenin, and beta-catenin is inhibited and their association with the cytoskeleton blocked. Overall cadherin expression, however, remains unchanged as demonstrated by studies with a pan-cadherin antibody. This is accompanied by an increase in expression of the cadherin cytoskeletal protein plakoglobin. Although the cells have tried to compensate for the loss of N-cadherin by up-regulation of another cadherin(s) and plakoglobin, this is unable to compensate for N-cadherin function. The data strongly suggest that N-cadherin and its associated cytoskeleton play an important role in the differentiation process that leads to the formation of the crystalline lens.

Enhancer-independent promoter activity of the mouse alphaB-crystallin/small heat shock protein gene in the lens and cornea of transgenic mice

The alphaB-crystallin/small heat shock protein gene is expressed very highly in the mouse eye lens and to a lesser extent in many other nonocular tissues, including the heart, skeletal muscle and brain. Previously we showed in transgenic mice that lens-specific alphaB-crystallin promoter activity is directed by a proximal promoter fragment (-164/+44) and that non-lens promoter activity depends on an upstream enhancer (-427/-259) composed of at least 5 cis-control elements. Here we have used truncated alphaB-crystallin promoter-CAT transgenes to test by biphasic CAT assays and/or histochemistry for specific expression in the cornea and lens. Deletion either of 87 bp (-427/-340) from the 5' end of the alphaB-crystallin enhancer or of the whole enhancer (-427/-258) abolished alphaB-crystallin promoter activity in all tissues except the lens and corneal epithelium when examined by the biphasic CAT assay in 4-5-week-old transgenic mice. These truncations also lowered promoter strength in the lens. The -426/+44-CAT, -339/+44-CAT and -164/+44-CAT (previously thought to be lens-specific in transgenic mice) transgenes were all expressed in the 4-6-week-old corneal epithelium when examined histochemically. Immunohistochemical staining confirmed the presence of endogenous alphaB-crystallin in the mature corneal epithelial cells. CAT gene expression driven by the alphaB-crystallin promoter with or without the enhancer was evident in the embryonic and 4-6-week-old lens. By contrast, activity of the alphaB-crystallin promoter/enhancer-CAT transgene was not detectable in the corneal epithelium before birth. Taken together, these results indicate that the intact enhancer of the alphaB-crystallin/small heat shock protein gene is required for promoter activity in all tissues tested except the lens and cornea.

Intercellular communication in the eye: clarifying the need for connexin diversity

In the vertebrate eye, virtually every cell type is directly coupled to its neighbors by intercellular channels present in gap junctions. Although these structures share the common property of allowing adjacent cells to directly exchange ions, second messengers and small metabolites, intercellular channels in the eye also play a specific role in distinct functions such as neuronal transmission at electrotonic synapses in the retina, and the maintenance of homeostasis in the avascular lens. The structural proteins comprising these channels, the connexins (Cx), are a multigene family of which many members are expressed in the eye, even in the same cell type. This molecular heterogeneity poses the crucial question of whether and how a diversity in gap junctional structural proteins influences intercellular communication in ocular tissues. This review will focus on two recent advances in the understanding of connexin diversity in regard to the eye. First, connexin knockouts have demonstrated that postnatal development and homeostasis in the lens requires multiple connexin proteins. Secondly, functional characterization of new connexins that are abundantly expressed in the retina has revealed biophysical properties that mimic those recorded from retinal neurons.

Different roles for Pax6 in the optic vesicle and facial epithelium mediate early morphogenesis of the murine eye

Chimaeric mice were made by aggregating Pax6(−/−) and wild-type mouse embryos, in order to study the interaction between the optic vesicle and the prospective lens epithelium during early stages of eye development. Histological analysis of the distribution of homozygous mutant cells in the chimaeras showed that the cell-autonomous removal of Pax6(−/−) cells from the lens, shown previously at E12.5, is nearly complete by E9.5. Most mutant cells are eliminated from an area of facial epithelium wider than, but including, the developing lens placode. This result suggests a role for Pax6 in maintaining a region of the facial epithelium that has the tissue competence to undergo lens differentiation. Segregation of wild-type and Pax6(−/−) cells occurs in the optic vesicle at E9.5 and is most likely a result of different adhesive properties of wild-type and mutant cells. Also, proximo-distal specification of the optic vesicle (as assayed by the elimination of Pax6(−/−) cells distally), is disrupted in the presence of a high proportion of mutant cells. This suggests that Pax6 operates during the establishment of patterning along the proximo-distal axis of the vesicle. Examination of chimaeras with a high proportion of mutant cells showed that Pax6 is required in the optic vesicle for maintenance of contact with the overlying lens epithelium. This may explain why Pax6(−/−) optic vesicles are inefficient at inducing a lens placode. Contact is preferentially maintained when the lens epithelium is also wild-type. Together, these results demonstrate requirements for functional Pax6 in both the optic vesicle and surface epithelia in order to mediate the interactions between the two tissues during the earliest stages of eye development.

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 novel lens epithelium gene, LEP503, is highly conserved in different vertebrate species and is developmentally regulated in postnatal rat lens

The development of the lens is dependent on the proliferation of lens epithelial cells and their differentiation into fiber cells near the lens bow/equator. Identification of genes specifically expressed in the lens epithelial cells and their functions may provide insight into molecular events that regulate the processes of lens epithelial cell differentiation. In this study, a novel lens epithelium gene product, LEP503, identified from rat by a subtractive cDNA cloning strategy was investigated in the genome organization, mRNA expression and protein localization. The genomic sequences for LEP503 isolated from rat, mouse and human span 1754 bp, 1694 bp and 1895 bp regions encompassing the 5'-flanking region, two exons, one intron and 3'-flanking region. All exon-intron junction sequences conform to the GT/AG rule. Both mouse and human LEP503 genes show very high identity (93% for mouse and 79% for human) to rat LEP503 gene in the exon 1 that contains an open reading frame coding for a protein of 61 amino acid residues with a leucine-rich domain. The deduced protein sequences also show high identity (91% between mouse and rat and 77% between human and rat). Western blot shows that LEP503 is present as a specific approximately 6.9 kDa band in the water-insoluble-urea-soluble fraction of lens cortex where lens epithelium is included. Immuno-staining shows that LEP503 is localized in the epithelial cells along the entire anterior surface of rat lens. Developmentally, LEP503 is expressed at a low level at newborn, and then the expression level increases by about ten-fold around postnatal day 14 and remains at this high level for about 25 days before it drops back to the low level by postnatal day 84. These data suggest that the LEP503 may be an important lens epithelial cell gene involving the processes of epithelial cell differentiation.

Regulation of mouse lens fiber cell development and differentiation by the Maf gene

Maf is a basic domain/leucine zipper domain protein originally identified as a proto-oncogene whose consensus target site in vitro, the T-MARE, is an extended version of an AP-1 site normally recognized by Fos and Jun. Maf and the closely related family members Neural retina leucine zipper (Nrl), L-Maf, and Krml1/MafB have been implicated in a wide variety of developmental and physiologic roles; however, mutations in vivo have been described only for Krml1/MafB, in which a loss-of-function causes abnormalities in hindbrain development due to failure to activate the Hoxa3 and Hoxb3 genes. We have used gene targeting to replace Maf coding sequences with those of lacZ, and have carried out a comprehensive analysis of embryonic expression and the homozygous mutant phenotype in the eye. Maf is expressed in the lens vesicle after invagination, and becomes highly upregulated in the equatorial zone, the site at which self-renewing anterior epithelial cells withdraw from the cell cycle and terminally differentiate into posterior fiber cells. Posterior lens cells in Maf(lacZ) mutant mice exhibit failure of elongation at embryonic day 11.5, do not express (α)A- and all of the (beta)-crystallin genes, and display inappropriately high levels of DNA synthesis. This phenotype partially overlaps with those reported for gene targeting of Prox1 and Sox1; however, expression of these genes is grossly normal, as is expression of Eya1, Eya2, Pax6, and Sox2. Recombinant Maf protein binds to T-MARE sites in the (alpha)A-, (beta)B2-, and (beta)A4-crystallin promoters but fails to bind to a point mutation in the (alpha)A-crystallin promoter that has been shown previously to be required for promoter function. Our results indicate that Maf directly activates many if not all of the (beta)-crystallin genes, and suggest a model for coordinating cell cycle withdrawal with terminal differentiation.

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.

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

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

Changes in cyclin dependent kinase expression and activity accompanying lens fiber cell differentiation

Previous studies from this laboratory have shown that differentiating lens fiber cells contain two active cyclin dependent kinases (Cdks), Cdk1 and Cdk5. The present study was undertaken to explore the expression and regulation of six additional members of the Cdk family (Cdk2, Cdk3, Cdk4, Cdk6, Cdk7 and Cdk8) during lens differentiation. Differentiating lens fiber cells were separated from lens epithelial cells by microdissection of developing rat lenses [embryonic day 16 (E16) to postnatal day 8 (P8)] and Cdk expression was assessed by RT-PCR and immunoblotting. Two Cdks (Cdk3 and Cdk6) were not expressed in lens fiber cells or epithelial cells during this developmental period. In the lens epithelium, we detected proteins and mRNAs corresponding to all other Cdks examined (Cdk2, Cdk4, Cdk7, Cdk8) throughout this developmental period. Epithelial cells showed significant Cdk2 activity, which decreased with developmental age, but no significant activity was detected for Cdk4, Cdk7, or Cdk8. Fiber cells contained all four Cdk proteins and the corresponding Cdk mRNAs except for Cdk2 mRNA. None of the Cdks examined showed significant kinase activity in fiber cells. Immunoprecipitates of Cdk2 and Cdk4 from fiber cells contained p57(kip2), supporting the view that this Cdk inhibitor blocks the activity of these Cdks in lens fibers. In contrast, p57(kip2)did not co-immunoprecipitate with Cdk5 from lens fibers. These findings suggest that the differential affinity of p57(kip2)for members of the Cdk family may provide a mechanism for specific regulation of individual Cdks during fiber cell differentiation.

Effect of age on the morphology and optical quality of the avian crystalline lens

The effect of age on the avian lens was examined using White Leghorn chickens of five age groups: hatchling (n =19), 7 day (n = 15), 34 week (n =10), 2 year (n =24), and 5 year (n =25). The chick lens grows steadily up to 34 weeks of age, after which, the rate of growth slows down. During growth, average focal length of the lens becomes longer. However, no significant changes were noted between 2 and 5 year old lenses. An age related increase in average lenticular focal length variability (FLV) was observed, revealing that the optical quality of the lens decreases with age. Scanning electron microscopy demonstrated that: (1) the suture region of the lens becomes more diffuse and less precise with age; (2) the central regions of younger lenses are oblate spheroids, while older lenses become more apple-shaped; (3) individual lens fibres in the young lens are crescent-shaped, while older lens fibres are square-bracket-like in shape; (4) the edges of individual lens fibres become more jagged and irregular with age; and (5) the layering of lens fibres is more disorderly in older lenses, in contrast to the parallel and organized layering of fibres in young lenses.

The gamma-crystallins and human cataracts: a puzzle made clearer

Despite the fact that cataracts constitute the leading cause of blindness worldwide, the mechanisms of lens opacification remain unclear. We recently mapped the aculeiform cataract to the gamma-crystallin locus (CRYG) on chromosome 2q33-35, and mutational analysis of the CRYG-genes cluster identified the aculeiform-cataract mutation in exon 2 of gamma-crystallin D (CRYGD). This mutation occurred in a highly conserved amino acid and could be associated with an impaired folding of CRYGD. During our study, we observed that the previously reported Coppock-like-cataract mutation, the first human cataract mutation, in the pseudogene CRYGE represented a polymorphism seen in 23% of our control population. Further analysis of the original Coppock-like-cataract family identified a missense mutation in a highly conserved segment of exon 2 of CRYGC. These mutations were not seen in a large control population. There is no direct evidence, to date, that up-regulation of a pseudogene causes cataracts. To our knowledge, these findings are the first evidence of an involvement of CRYGC and support the role of CRYGD in human cataract formation.

Members of the bcl-2 and caspase families regulate nuclear degeneration during chick lens fibre differentiation

The optical clarity of the lens is ensured by the programmed removal of nuclei and other organelles from the lens fibre cells during development. The morphology of the degenerating nuclei is similar to that observed during apoptosis and is accompanied by DNA fragmentation. Proteins encoded by the bcl-2 proto-oncogene family are important in either promoting or inhibiting apoptosis, and caspases are involved in downstream proteolytic events. Here, the expression of bcl-2 family members (bcl-2, bax, bad, and bcl-x(s/l)) and caspases-1, -2, -3, -4, and -6 was investigated through a range of stages of chick lens development using immunocytochemistry, Western blotting, and affinity labelling for caspases using biotinylated caspase inhibitors. Using differentiating lens epithelial cell cultures, it was demonstrated that the addition to cultures of synthetic peptide inhibitors of caspases -1, -2, -4, -6, and -9 brought about a 50-70% reduction in the number of degenerating nuclei per unit area of culture, as assessed by image analysis. These effects were comparable to those seen when general inhibitors of caspases were added to cultures. On the other hand, inhibitors of caspases-3 and -8 were not effective in significantly reducing the number of TUNEL-labelled nuclei. Expression of the caspase substrates poly(ADP-ribose) polymerase (PARP) and the 45-kDa subunit of DNA fragmentation factor (DFF 45) was also observed in the developing lens. Western blots of cultures to which caspase inhibitors were added revealed alterations in the PARP cleavage pattern, but not in that of DFF. These results demonstrate a role for members of the bcl-2 family and caspases in the degeneration of lens fibre cell nuclei during chick secondary lens fibre development and support the proposal that this process has many characteristics in common with apoptosis.

Disruption of retinoblastoma protein family function by human papillomavirus type 16 E7 oncoprotein inhibits lens development in part through E2F-1

Complexes between the retinoblastoma protein (pRb) and the transcription factor E2F-1 are thought to be important for regulating cell proliferation. We have shown previously that the E7 oncoprotein from human papillomavirus type 16, dependent upon its binding to pRb proteins, induces proliferation, disrupts differentiation, and induces apoptosis when expressed in the differentiating, or fiber, cells of the ocular lenses in transgenic mice. Mice that carry a null mutation in E2F-1 do not exhibit any defects in proliferation and differentiation in the lens. By examining the lens phenotype in mice that express E7 on an E2F-1 null background, we now show genetic evidence that E7's ability to alter the fate of fiber cells is partially dependent on E2F-1. On the other hand, E2F-1 status does not affect E7-induced proliferation in the undifferentiated lens epithelium. These data provide genetic evidence that E2F-1, while dispensible for normal fiber cell differentiation, is one mediator of E7's activity in vivo and that the requirement for E2F-1 is context dependent. These data suggest that an important role for pRb-E2F-1 complex during fiber cell differentiation is to negatively regulate cell cycle progression, thereby allowing completion of the differentiation program to occur.

Regulation of lens fiber cell differentiation by transcription factor c-Maf

To elucidate the regulatory mechanisms underlying lens development, we searched for members of the large Maf family, which are expressed in the mouse lens, and found three, c-Maf, MafB, and Nrl. Of these, the earliest factor expressed in the lens was c-Maf. The expression of c-Maf was most prominent in lens fiber cells and persisted throughout lens development. To examine the functional contribution of c-Maf to lens development, we isolated genomic clones encompassing the murine c-maf gene and carried out its targeted disruption. Insertion of the beta-galactosidase (lacZ) gene into the c-maf locus allowed visualization of c-Maf accumulation in heterozygous mutant mice by staining for LacZ activity. Homozygous mutant embryos and newborns lacked normal lenses. Histological examination of these mice revealed defective differentiation of lens fiber cells. The expression of crystallin genes was severely impaired in the c-maf-null mutant mouse lens. These results demonstrate that c-Maf is an indispensable regulator of lens differentiation during murine development.

Lens cytoskeleton and transparency: a model

The function of the cytoskeleton in lens was first considered when cytoplasmic microtubules were observed in elongating fibre cells of the chick lens nearly 40 years ago. Since that time, tubulin, actin, vimentin and intermediate filaments have been identified and found to function in mitosis, motility and cellular morphology during lens cell differentiation. A role for the cytoskeleton in accommodation has been proposed and modification of the cytoskeletal proteins has been observed in several cataract models. Recently, a progressive increase in protein aggregation and lens opacification was found to correspond with the loss of cytoskeletal protein in the selenite model for cataract. In the present report a model is proposed for the role of tubulin, actin, vimentin, spectrin and the lens-specific filaments, filensin and CP49, in the establishment and maintenance of transparent lens cell structure.

Transcription factors for lens development assessed in vivo

Lens-cell differentiation occurs at a fairly early stage of embryogenesis and results in very simple tissue architecture. These features allow the embryonic lens to provide a paradigm of tissue development starting from tissue induction to tissue maturation. Not only have a number of transcription factors participating in lens development been identified but their actual functions are now assessed by modern approaches utilizing genetic and tissue manipulations of embryos.

mRNA and protein localization of the IGF system during mouse embryonic development in areas with apoptosis

We analysed mRNA and protein localization of the IGF system components in regions with apoptosis during mouse development between 9.5 and 13.5 days post coitum. A spatio-temporal relationship between these expression patterns and the onset of apoptosis in specific areas was sought. The IGFBP mRNA and protein expression patterns were tissue-specific. In most tissues, mRNA expression patterns colocalized with protein localization. Discrepancies between mRNA and protein detection were found in, for example, lens, neural layer of the retina, whiskers and somites. Localization of the IGFs, the type I IGF receptor and IGFBP-2 correlated well with cell death regions. When these genes were expressed no apoptosis occurred and vice versa. Correlation of IGFBP-3, -4 and -5 with apoptosis regions was noticed only at 13.5 days post coitum. In eye muscles, whiskers and somites, the expression of IGF system components preceded the occurrence of apoptotic cells. When IGF-I expression ceased, apoptosis occurred in these areas. In conclusion, our results suggest that IGF-I, the type I IGF receptor and IGFBP-2 inhibit apoptosis. In contrast, IGFBP-3, -4 and -5 may stimulate apoptosis by trapping the IGFs. Tissue-specific modulation of IGF protective action against apoptosis by the different IGFBPs during mouse embryonal development may contribute to organ specific morphology.

Genetic diseases and gene knockouts reveal diverse connexin functions

Intercellular channels present in gap junctions allow cells to share small molecules and thus coordinate a wide range of behaviors. Remarkably, although junctions provide similar functions in all multicellular organisms, vertebrates and invertebrates use unrelated gene families to encode these channels. The recent identification of the invertebrate innexin family opens up powerful genetic systems to studies of intercellular communication. At the same time, new information on the physiological roles of vertebrate connexins has emerged from genetic studies. Mutations in connexin genes underlie a variety of human diseases, including deafness, demyelinating neuropathies, and lens cataracts. In addition, gene targeting of connexins in mice has provided new insights into connexin function and the significance of connexin diversity.

AP-2alpha transcription factor is required for early morphogenesis of the lens vesicle

AP-2 transcription factors are a family of retinoic acid-responsive genes, which are involved in complex morphogenetic processes. In the current study, we determine the requirement for AP-2alpha in early morphogenesis of the eye by examining the nature of the ocular defects in AP-2alpha null and chimeric mice. AP-2alpha null embryos exhibited ocular phenotypes ranging from a complete lack of eyes (anophthalmia) to defects in the developing lens involving a persistent adhesion of the lens to the overlying surface ectoderm. Two genes involved in lens development and differentiation, Pax6 and MIP26 were also misexpressed. AP-2alpha mutants also exhibited defects in the optic cup consisting of transdifferentiation of the dorsal retinal pigmented epithelium into neural retina and the absence of a defined ganglion cell layer. Newly generated chimeric embryos consisting of a population of AP-2alpha-/- and AP-2alpha+/+ cells exhibit ocular defects similar to those seen in the knockout embryos. Immunolocalization of AP-2 proteins (alpha, beta, and gamma) to the normal developing eye revealed both unique and overlapping expression patterns, with AP-2alpha expressed in a number of the ocular tissues that exhibited defects in the mutants, including the developing lens where AP-2alpha is uniquely expressed. Together these findings demonstrate a requirement for AP-2alpha in early morphogenesis of the eye.

TGF-beta elicits fibronectin secretion and proliferation in cultured chick lens epithelial cells

PURPOSE

To determine if the cataract forming influence of TGF-beta on lens cells is due to its effects on the ECM.

METHODS

Primary cultures of chick lens annular pad cells were exposed to TGF-beta and various exogenously supplied components of the lens capsule. Proliferative response were measured through tritiated thymidine incorporation into DNA. Cell spreading accompanying increased matrix interactions and growth was monitored with phase contrast microscopy. ECM proteins were detected in culture media and as deposited matrices with Western blotting and silver staining. TGF-beta receptors were identified with Western blotting.

RESULTS

Chick lens cells were shown to express type I and II TGF-beta receptors. TGF-beta stimulated cell growth and ECM production particularly with regard to fibronectin. Fibronectin was secreted into the culture medium and deposited onto plastic substrates. Plating cells on ECM components found in the lens capsule further increased their growth in response to TGF-beta.

CONCLUSIONS

These results indicate that TGF-beta may have a normal function in the lens regulating capsular protein production. The potent stimulation of lens cell growth by TGF-beta may be due to mis-regulated production of lens capsular proteins not normally found in great abundance.

Cyclin D- and E-dependent kinases and the p57(KIP2) inhibitor: cooperative interactions in vivo

This study examines in vivo the role and functional interrelationships of components regulating exit from the G1 resting phase into the DNA synthetic (S) phase of the cell cycle. Our approach made use of several key experimental attributes of the developing mouse lens, namely its strong dependence on pRb in maintenance of the postmitotic state, the down-regulation of cyclins D and E and up-regulation of the p57(KIP2) inhibitor in the postmitotic lens fiber cell compartment, and the ability to target transgene expression to this compartment. These attributes provide an ideal in vivo context in which to examine the consequences of forced cyclin expression and/or of loss of p57(KIP2) inhibitor function in a cellular compartment that permits an accurate quantitation of cellular proliferation and apoptosis rates in situ. Here, we demonstrate that, despite substantial overlap in cyclin transgene expression levels, D-type and E cyclins exhibited clear functional differences in promoting entry into S phase. In general, forced expression of the D-type cyclins was more efficient than cyclin E in driving lens fiber cells into S phase. In the case of cyclins D1 and D2, ectopic proliferation required their enhanced nuclear localization through CDK4 coexpression. High nuclear levels of cyclin E and CDK2, while not sufficient to promote efficient exit from G1, did act synergistically with ectopic cyclin D/CDK4. The functional differences between D-type and E cyclins was most evident in the p57(KIP2)-deficient lens wherein cyclin D overexpression induced a rate of proliferation equivalent to that of the pRb null lens, while overexpression of cyclin E did not increase the rate of proliferation over that induced by the loss of p57(KIP2) function. These in vivo analyses provide strong biological support for the prevailing view that the antecedent actions of cyclin D/CDK4 act cooperatively with cyclin E/CDK2 and antagonistically with p57(KIP2) to regulate the G1/S transition in a cell type highly dependent upon pRb.

Normal differentiation of cultured lens cells after inhibition of gap junction-mediated intercellular communication

The cells of the vertebrate lens are linked to each other by gap junctions, clusters of intercellular channels that mediate the direct transfer of low-molecular-weight substances between the cytosols of adjoining cells. Although gap junctions are detectable in the unspecialized epithelial cells that comprise the anterior face of the organ, both their number and size are greatly increased in the secondary fiber cells that differentiate from them at the lens equator. In other organs, gap junctions have been shown to play an important role in tissue development and differentiation. It has been proposed, although not experimentally tested, that this may be true in the lens as well. To investigate the function of gap junctions in the development of the lens, we have examined the effect of the gap junction blocker 18beta-glycyrrhetinic acid (betaGA) on the differentiation of primary cultures (both dissociated cell-derived monolayers and central epithelium explants) of embryonic chick lens epithelial cells. We found that betaGA greatly reduced gap junction-mediated intercellular transfer of Lucifer yellow and biocytin throughout the 8-day culture period. betaGA did not, however, affect the differentiation of these cells into MP28-expressing secondary fibers. Furthermore, inhibition of gap junctions had no apparent effect on either of the two other types of intercellular (adherens and tight) junctions present in the lens. We conclude that the high level of gap junctional intercellular communication characteristic of the lens equator in vivo is not required for secondary fiber formation as assayed in culture. Up-regulation of gap junctions is therefore likely to be a consequence rather than a cause of lens fiber differentiation and may primarily play a role in lens physiology.

Activation of the Jak-STAT-signaling pathway in embryonic lens cells

Previous studies showed that lens epithelial cells proliferate rapidly in the embryo and that a lens mitogen, most likely derived from the blood, is present in the anterior chamber of the embryonic eye (Hyatt, G. A., and Beebe, D. C., Development 117, 701-709, 1993). Messenger RNAs for several growth factor receptors have been identified in embryonic lens epithelial cells. We tested several growth factors that are ligands for these receptors for their ability to maintain lens cell proliferation. Embryo serum, PDGF, GM-CSF, and G-CSF maintained lens cell proliferation, but NGF, VEGF, and HGF did not. This and a previous study (Potts, J. D., Harocopos, G. J., and Beebe, D. C., Curr. Eye Res. 12, 759-763, 1993) detected members of the Janus kinase family (Jaks) in the developing lens. Because Jaks are central players in the Jak-STAT-signaling pathway, we identified STAT proteins in the lens and tested whether they were phosphorylated in response to mitogens. STAT1 and STAT3, but not STAT 5 were detected in chicken embryo lens epithelial cells. Only STAT3 was found in terminally differentiated lens fiber cells. STAT1 and STAT3 were phosphorylated in lens cells analyzed immediately after removal from the embryo and when lens epithelial explants were treated with embryo serum, PDGF, or GM-CSF, but not with NGF. Chicken embryo vitreous humor or IGF-1, factors that stimulate lens cell differentiation, but not proliferation, did not cause STAT phosphorylation. When lens epithelial cells were cultured for 4 h in unsupplemented medium, STAT1 and STAT3 declined to nearly undetectable levels. Treatment with PDGF or embryo serum for an additional 15 min restored STAT1 and -3 levels. This recovery was blocked by cycloheximide, but not actinomycin D, suggesting that STAT levels are regulated at the level of translation. STAT levels were maintained in epithelial explants by lens mitogens, but not by factors that stimulated lens fiber differentiation. Both factors that stimulated lens cell proliferation and those that caused fiber differentiation protected cultured lens epithelial cells from apoptosis. These data suggest that the factor(s) responsible for lens cell proliferation in vivo activates the Jak-STAT-signaling pathway. They also indicate that growth factors maintain STAT protein levels in lens epithelial cells by promoting the translation of STAT mRNA, an aspect of STAT regulation that has not been described previously. Signaling by most of the growth factors and cytokines known to activate the Jak-STAT pathway has been disrupted in mice by mutation or targeted deletion. Consideration of the phenotypes of these mice suggests that the factor responsible for lens cell proliferation in vivo may be a growth factor or cytokine that has not yet been described.

Targeted ablation of connexin50 in mice results in microphthalmia and zonular pulverulent cataracts

In the ocular lens, gap junctional communication is a key component of homeostatic mechanisms preventing cataract formation. Gap junctions in rodent lens fibers contain two known intercellular channel-forming proteins, connexin50 (Cx50) and Cx46. Since targeted ablation of Cx46 has been shown to cause senile-type nuclear opacities, it appears that Cx50 alone cannot meet homeostatic requirements. To determine if lens pathology arises from a reduction in levels of communication or the loss of a connexin-specific function, we have generated mice with a targeted deletion of the Cx50 gene. Cx50-null mice exhibited microphthalmia and nuclear cataracts. At postnatal day 14 (P14), Cx50-knockout eyes weighed 32% less than controls, whereas lens mass was reduced by 46%. Cx50-knockout lenses also developed zonular pulverulent cataracts, and lens abnormalities were detected by P7. Deletion of Cx50 did not alter the amounts or distributions of Cx46 or Cx43, a component of lens epithelial junctions. In addition, intercellular passage of tracers revealed the persistence of communication between all cell types in the Cx50-knockout lens. These results demonstrate that Cx50 is required not only for maintenance of lens transparency but also for normal eye growth. Furthermore, these data indicate that unique functional properties of both Cx46 and Cx50 are required for proper lens development.

Cooperation between the Cdk inhibitors p27(KIP1) and p57(KIP2) in the control of tissue growth and development

Cell cycle exit is required for terminal differentiation of many cell types. The retinoblastoma protein Rb has been implicated both in cell cycle exit and differentiation in several tissues. Rb is negatively regulated by cyclin-dependent kinases (Cdks). The main effectors that down-regulate Cdk activity to activate Rb are not known in the lens or other tissues. In this study, using multiple mutant mice, we show that the Cdk inhibitors p27(KIP1) and p57(KIP2) function redundantly to control cell cycle exit and differentiation of lens fiber cells and placental trophoblasts. These studies demonstrate that p27(KIP1) and p57(KIP2) are critical terminal effectors of signal transduction pathways that control cell differentiation.

Dual roles for Pax-6: a transcriptional repressor of lens fiber cell-specific beta-crystallin genes

It has been demonstrated previously that Pax-6, a paired domain (PD)/homeodomain (HD) transcription factor critical for eye development, contributes to the activation of the alphaB-, alphaA-, delta1-, and zeta-crystallin genes in the lens. Here we have examined the possibility that the inverse relationship between the expression of Pax-6 and beta-crystallin genes within the developing chicken lens reflects a negative regulatory role of Pax-6. Cotransfection of a plasmid containing the betaB1-crystallin promoter fused to the chloramphenicol acetyltransferase reporter gene and a plasmid containing the full-length mouse Pax-6 coding sequences into primary embryonic chicken lens epithelial cells or fibroblasts repressed the activity of this promoter by as much as 90%. Pax-6 constructs lacking the C-terminal activation domain repressed betaB1-crystallin promoter activity as effectively as the full-length protein, but the PD alone or Pax-6 (5a), a splice variant with an altered PD affecting its DNA binding specificity, did not. DNase footprinting analysis revealed that truncated Pax-6 (PD+HD) binds to three regions (-183 to -152, -120 to -48, and -30 to +1) of the betaB1-crystallin promoter. Earlier experiments showed that the betaB1-crystallin promoter sequence from -120 to -48 contains a cis element (PL2 at -90 to -76) that stimulates the activity of a heterologous promoter in lens cells but not in fibroblasts. In the present study, we show by electrophoretic mobility shift assay and cotransfection that Pax-6 binds to PL2 and represses its ability to activate promoter activity; moreover, mutation of PL2 eliminated binding by Pax-6. Taken together, our data indicate that Pax-6 (via its PD and HD) represses the betaB1-crystallin promoter by direct interaction with the PL2 element. We thus suggest that the relatively high concentration of Pax-6 contributes to the absence of betaB1-crystallin gene expression in lens epithelial cells and that diminishing amounts of Pax-6 in lens fiber cells during development allow activation of this gene.

Molecular regulators involved in vertebrate eye development

Development of the eye can be subdivided into three phases. The first phase is the formation of the major structures of the eye by the processes of induction and regional specification. The second is the maturation of these structures to form the functional eye, and the third phase is the formation of neuronal connections between retina and the optic tectum. These processes are tightly regulated by signalling cascades that direct axonal outgrowth, cellular proliferation and differentiation. Some members of these signalling cascades have been identified in recent studies. These include secreted factors which transmit signals extracellularly, and receptors and transcription factors which are members of intracellular signalling pathways that respond to extracellular signals. This review summarizes the recent research that has implicated these factors in playing a role in eye development on the basis of functional or expression criteria.

Transcription factor CP2 is essential for lens-specific expression of the chicken alphaA-crystallin gene

BACKGROUND

Lens-specific transcriptional activation of the chicken alphaA-crystallin gene is controlled by the distal and proximal enhancers, alphaCE1 and alphaCE2, respectively. Analysis using specific monoclonal antibodies against purified alphaCE1-binding factor alphaCEF1 revealed that alphaCEF1 is composed of two distinct subunits.

RESULTS

We have demonstrated that one of the subunits of alphaCEF1 is encoded by chicken ubiquitous transcription factor CP2 (cCP2), which is homologous to mouse CP2, and human CP2/LBP-1/LSF-1. Electrophoretic mobility shift assays and cross-linking experiments showed that alphaCEF1 and bacterially expressed cCP2 form a tetramer. Overexpression of cCP2 activates transcription through alphaCE1, but a mutant cCP2 lacking the DNA-binding domain reduced the transcription to basal levels. Although cCP2 binds to the CP2 template from the mouse alpha-globin promoter, it fails to promote transcription through this template. Element substitution experiments between alphaCE1 and the CP2 template revealed that the lens-specific enhancer activity of alphaCE1 is due to the 6 bp sequence (-139/-134; lens-specific element (LSE)) adjacent to the 3' of the cCP2 binding site within alphaCE1.

CONCLUSION

We have shown that the tetrameric transcription factor cCP2 is essential for lens-specific transcription of the chicken alphaA-crystallin gene, although it is ubiquitously expressed. We propose a model where cCP2 cooperates with a putative lens-specific factor which binds to LSE.

Involvement of Sox1, 2 and 3 in the early and subsequent molecular events of lens induction

Activation of the first lens-specific gene of the chicken, delta 1-crystallin, is dependent on a group of lens nuclear factors, deltaEF2, interacting with the delta1-crystallin minimal enhancer, DC5. One of the deltaEF2 factors was previously identified as SOX2. We show that two related SOX proteins, SOX1 and SOX3, account for the remaining members of deltaEF2. Activation of the DC5 enhancer is dependent on their C-terminal domains. Expression of Sox1-3 in the eye region during lens induction was studied in comparison with Pax6 and delta1-crystallin. Pax6, known to be required for the inductive response of the ectoderm, is broadly expressed in the lateral head ectoderm from before lens induction. After tight association of the optic vesicle (around stage 10–11, 40 hours after egg incubation), expression of Sox2 and Sox3 is activated in the vesicle-facing ectoderm at stage 12 (44 hours). These cells, expressing together Pax6 and Sox2/3, subsequently give rise to the lens, beginning with formation of the lens placode and expression of delta-crystallin at stage 13 (48 hours). Sox1 then starts to be expessed in the lens-forming cells at stage 14. When the prospective retina area of the neural plate was unilaterally ablated at stage 7, expression of Sox2/3 was lost in the side of lateral head ectoderm lacking the optic cup, implying that an inductive signal from the optic cup activates Sox2/3 expression. In the mouse embryonic lens, this subfamily of Sox genes is expressed in an analogous fashion, although Sox3 transcripts have not been detected and Sox2 expression is down-regulated when Sox1 is activated. In ectodermal tissues of the chicken embryo, delta -crystallin expression occurs in a few ectopic sites. These are always characterized by overlapping expression of Sox2/3 and Pax6. Thus, an essential molecular event in lens induction is the ‘turning on’ of the transcriptional regulators SOX2/3 in the Pax6-expressing ectoderm and these SOX proteins activate crystallin gene expression. Continued activity, especially of SOX1, is then essential for further development of the lens.

Structure, development and function of cytoskeletal elements in non-neuronal cells of the human eye

The cytoskeleton, of which the main components in the human eye are actin microfilaments, intermediate filaments and microtubules with their associated proteins, is essential for the normal growth, maturation, differentiation, integrity and function of its cells. These components interact with intra- and extracellular environment and each other, and their profile frequently changes during development, according to physiologic demands, and in various diseases. The ocular cytoskeleton is unique in many ways. A special pair of cytokeratins, CK 3 and 12, has apparently evolved only for the purposes of the corneal epithelium. However, other cytokeratins such as CK 4, 5, 14, and 19 are also important for the normal ocular surface epithelia, and other types may be acquired in keratinizing diseases. The intraocular tissues, which have a relatively simple cytoskeleton consisting mainly of vimentin and simple epithelial CK 8 and 18, differ in many details from extraocular ones. The iris and lens epithelium characteristically lack cytokeratins in adults, and the intraocular muscles all have a cytoskeletal profile of their own. The dilator of the iris contains vimentin, desmin and cytokeratins, being an example of triple intermediate filament expression, but the ciliary muscle lacks cytokeratin and the sphincter of the iris is devoid even of vimentin. Conversion from extraocular-type cytoskeletal profile occurs during fetal life. It seems that posttranslational modification of cytokeratins in the eye may also differ from that of extraocular tissues. So far, it has not been possible to reconcile the cytoskeletal profile of intraocular tissues with their specific functional demands, but many theories have been put forward. Systematic search for cytoskeletal elements has also revealed novel cell populations in the human eye. These include transitional cells of the cornea that may represent stem cells on migration, myofibroblasts of the scleral spur and juxtacanalicular tissue that may modulate aqueous outflow, and subepithelial matrix cells of the ciliary body and myofibroblasts of the choroid that may both participate in accommodation. In contrast to the structure and development of the ocular cytoskeleton, changes that take place in ocular disease have not been analysed systematically. Nevertheless, potentially meaningful changes have already been observed in corneal dystrophies (Meesmann's dystrophy, posterior polymorphous dystrophy and iridocorneal endothelial syndrome), degenerations (pterygium) and inflammatory diseases (Pseudomonas keratitis), in opacification of the lens (anterior subcapsular and secondary cataract), in diseases characterized by proliferation of the retinal pigment epithelium (macular degeneration and proliferative vitreoretinopathy), and in intraocular tumours (uveal melanoma). In particular, upregulation of alpha-smooth muscle actin seems to be a relatively general response typical of spreading and migrating corneal stromal and lens epithelial cells, trabecular cells and retinal pigment epithelial cells.

Six3, a medaka homologue of the Drosophila homeobox gene sine oculis is expressed in the anterior embryonic shield and the developing eye

The conserved transcription factor Pax6 is essential for eye development in Drosophila and mammals (Hill, R.E., Favor, J., Hogan, B.L.M., Ton, C.C.T., Saunders, G.F., Hanson, I.M., Prosser, J., Jordan, T., Hastie, N.D., van Heyningen, V., 1991. Mouse small eye results from mutations in a paired-like homeobox containing gene. Nature 354, 522-525; Ton, C., Hirvonen, H., Miwa, H., Weil, M., Monaghan, P., Jordan, T., van Heyningen, V., Hastie, N., Meijers-Heijboer, H., Drechsler, M., Royer-Pokora, B., Collins, F., Swaroop, A., Strong, L.C., Saunders, G.F., 1991. Positional cloning and characterization of a paired box- and homeobox-containing gene from the aniridia region. Cell 6, 1059-1074; Matsuo, T., Osumi-Yamashita, N., Noji, S., Ohuchi, H., Koyama, E., Myokai, F., Matsuo, N., Toniguchi, S., Dari, H., Jseki, S., Ninomiya, Y., Fujiwara, M., Watanabe, T., Eto, K., 1993. A mutation at the Pax-6 gene in rat small eye is associated with impaired migration of midbrain crest cells. Nature genet. 3, 299-304; Quiring, R., Walldorf, U., Kloter, U., Gehring, W.J., 1994. Homology of the eyeless gene of Drosophila to the small eye gene in mice and aniridia in humans. Science 265, 785-789). These findings led to the hypothesis that additional genes involved in invertebrate and vertebrate eye development are structurally and functionally conserved (Halder, G., Callaerts, P., Gehring, W.J., 1995. New perspectives on eye evolution. Curr. Opin. Gen. Dev. 5, 602-609; Quiring, R., Walldorf, U., Kloter, U., Gehring, W.J., 1994. Homology of the eyeless gene of Drosophila to the small eye gene in mice and aniridia in humans. Science 265, 785-789). Candidates for such conserved genes are the Drosophila homeobox gene sine oculis (Cheyette, B.N.R., Green, P.J., Martin, K., Garren, H., Hartenstein, V., Zipursky, S.L., 1994. The Drosophila sine oculis locus encodes a homeodomain-containing protein required for the development of the entire visual system. Neuron l2, 977-996) and its murine homologue Six3 (Oliver, G., Mailhos, A., Wehr, R., Copeland, N.G., Jenkins, N.A., Gruss, P., 1995. Six3, a murine homologue of the sine oculis gene, demarcates the most anterior border of the developing neural plate and is expressed during eye development. Development 121, 4045-4055). sine oculis (so) is essential for the development of the larval and adult visual system (Cheyette, B.N.R., Green, P.J., Martin, K., Garren, H., Hartenstein, V., Zipursky, S.L., 1994. The Drosophila sine oculis locus encodes a homeodomain-containing protein required for the development of the entire visual system. Neuron l2, 977-996). Six3 is expressed in the anterior neural plate and optic vesicles, lens, olfactory placodes and ventral forebrain (Oliver, G., Mailhos, A., Wehr, R., Copeland, N.G., Jenkins, N.A., Gruss, P., 1995. Six3, a murine homologue of the sine oculis gene, demarcates the most anterior border of the developing neural plate and is expressed during eye development. Development 121, 4045-4055). Overexpression of mouse Six3 gene in medaka fish embryos (Orvzias latipes) results in the formation of an ectopic lens, indicating that Six3 activity can trigger the genetic pathway leading to lens formation (Oliver, G., Loosli, F., Koster, R., Wittbrodt, J., Gruss, P., 1996. Ectopic lens induction in fish in response to the murine homeobox gene Six3. Mech. Dev. 60, 233-239). We isolated the medaka Six3 homologue and analyzed its expression pattern in the medaka embryo. It is expressed initially in the anterior embryonic shield and later in the developing eye and prosencephalon. The early localized expression of Six3 suggests a role in the regionalization of the rostral head.

Disregulation of ocular morphogenesis by lens-specific expression of FGF-3/int-2 in transgenic mice

FGF-3, originally named int-2, was discovered as an oncogene frequently activated in mammary carcinomas resulting from the chromosomal integration of the mouse mammary tumor virus (MMTV). Int-2 was later designated FGF-3 based on sequence homology with other members of the fibroblast growth factor (FGF) family. FGF-1 is the prototypical member of the FGF family, and is the only family member which activates all known FGF receptor isoforms. Transgenic mice expressing in the lens a form of FGF-1 engineered to be secreted show premature differentiation of the entire lens epithelium. In contrast, transgenic mice engineered to secrete FGF-2 in the lens do not undergo premature differentiation of the lens epithelium (C. M. Stolen et al., 1997, Development 124, 4009-4017). To further assess the roles of FGFs and FGF receptors in lens development, the alpha A-crystallin promoter was used to target expression of FGF-3 to the developing lens of transgenic mice. The expression of FGF-3 in the lens rapidly induced epithelial cells throughout the lens to elongate and to express fiber cell-specific proteins including MIP and beta-crystallins. This premature differentiation of the lens epithelium was followed by the degeneration of the entire lens. Since FGF-1 and FGF-3 can both activate one FGF receptor isoform (FGFR2 IIIb) that is not activated by FGF-2, these results suggest that activation of FGFR2 IIIb is sufficient to induce fiber cell differentiation throughout the lens epithelium in vivo. Furthermore, transgenic lens cells expressing FGF-3 were able to induce the differentiation of neighboring nontransgenic lens epithelial cells in chimeric mice. Expression of FGF-3 in the lens also resulted in developmental alterations of the eyelids, cornea, and retina, and in the most severely affected transgenic lines, the postnatal appearance of intraocular glandular structures.

Induction of lens differentiation by activation of a bZIP transcription factor, L-Maf

After the vertebrate lens is induced from head ectoderm, lens-specific genes are expressed. Transcriptional regulation of the lens-specific alphaA-crystallin gene is controlled by an enhancer element, alphaCE2. A gene encoding an alphaCE2-binding protein, L-maf(lens-specific maf), was isolated. L-maf expression is initiated in the lens placode and is restricted to lens cells. The gene product L-Maf regulates the expression of multiple genes expressed in the lens, and ectopic expression of this transcription factor converts chick embryonic ectodermal cells and cultured cells into lens fibers. Thus, vertebrate lens induction and differentiation can be triggered by the activation of L-Maf.