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

Jack of all trades, master of each: the diversity of fibroblast growth factor signalling in eye development

A central question in development biology is how a limited set of signalling pathways can instruct unlimited diversity of multicellular organisms. In this review, we use three ocular tissues as models of increasing complexity to present the astounding versatility of fibroblast growth factor (FGF) signalling. In the lacrimal gland, we highlight the specificity of FGF signalling in a one-dimensional model of budding morphogenesis. In the lens, we showcase the dynamics of FGF signalling in altering functional outcomes in a two-dimensional space. In the retina, we present the prolific utilization of FGF signalling from three-dimensional development to homeostasis. These examples not only shed light on the cellular basis for the perfection and complexity of ocular development, but also serve as paradigms for the diversity of FGF signalling.

Insights into Bone Morphogenetic Protein-(BMP-) Signaling in Ocular Lens Biology and Pathology

Bone morphogenetic proteins (BMPs) are a diverse class of growth factors that belong to the transforming growth factor-beta (TGFβ) superfamily. Although originally discovered to possess osteogenic properties, BMPs have since been identified as critical regulators of many biological processes, including cell-fate determination, cell proliferation, differentiation and morphogenesis, throughout the body. In the ocular lens, BMPs are important in orchestrating fundamental developmental processes such as induction of lens morphogenesis, and specialized differentiation of its fiber cells. Moreover, BMPs have been reported to facilitate regeneration of the lens, as well as abrogate pathological processes such as TGFβ-induced epithelial-mesenchymal transition (EMT) and apoptosis. In this review, we summarize recent insights in this topic and discuss the complexities of BMP-signaling including the role of individual BMP ligands, receptors, extracellular antagonists and cross-talk between canonical and non-canonical BMP-signaling cascades in the lens. By understanding the molecular mechanisms underlying BMP activity, we can advance their potential therapeutic role in cataract prevention and lens regeneration.

FGFR and PTEN signaling interact during lens development to regulate cell survival

Lens epithelial cells express many receptor tyrosine kinases (RTKs) that stimulate PI3K-AKT and RAS-RAF-MEK-ERK intracellular signaling pathways. These pathways ultimately activate the phosphorylation of key cellular transcription factors and other proteins that control proliferation, survival, metabolism, and differentiation in virtually all cells. Among RTKs in the lens, only stimulation of fibroblast growth factor receptors (FGFRs) elicits a lens epithelial cell to fiber cell differentiation response in mammals. Moreover, although the lens expresses three different Fgfr genes, the isolated removal of Fgfr2 at the lens placode stage inhibits both lens cell survival and fiber cell differentiation. Phosphatase and tensin homolog (PTEN), commonly known as a tumor suppressor, inhibits ERK and AKT activation and initiates both apoptotic pathways, and cell cycle arrest. Here, we show that the combined deletion of Fgfr2 and Pten rescues the cell death phenotype associated with Fgfr2 loss alone. Additionally, Pten removal increased AKT and ERK activation, above the levels of controls, in the presence or absence of Fgfr2. However, isolated deletion of Pten failed to stimulate ectopic fiber cell differentiation, and the combined deletion of Pten and Fgfr2 failed to restore differentiation-specific Aquaporin0 and DnaseIIβ expression in the lens fiber cells.

Loss of Dlg-1 in the mouse lens impairs fibroblast growth factor receptor signaling

Coordination of cell proliferation, differentiation and survival is essential for normal development and maintenance of tissues in the adult organism. Growth factor receptor tyrosine kinase signaling pathways and planar cell polarity pathways are two regulators of many developmental processes. We have previously shown through analysis of mice conditionally null in the lens for the planar cell polarity gene (PCP), Dlg-1, that Dlg-1 is required for fiber differentiation. Herein, we asked if Dlg-1 is a regulator of the Fibroblast growth factor receptor (Fgfr) signaling pathway, which is known to be required for fiber cell differentiation. Western blot analysis of whole fiber cell extracts from control and Dlg-1 deficient lenses showed that levels of the Fgfr signaling intermediates pErk, pAkt, and pFrs2α, the Fgfr target, Erm, and the fiber cell specific protein, Mip26, were reduced in the Dlg-1 deficient fiber cells. The levels of Fgfr2 were decreased in Dlg-1 deficient lenses compared to controls. Conversely, levels of Fgfr1 in Dlg-1 deficient lenses were increased compared to controls. The changes in Fgfr levels were found to be specifically in the triton insoluble, cytoskeletal associated fraction of Dlg-1 deficient lenses. Immunofluorescent staining of lenses from E13.5 embryos showed that expression levels of pErk were reduced in the transition zone, a region of the lens that exhibits PCP, in the Dlg-1 deficient lenses as compared to controls. In control lenses, immunofluorescent staining for Fgfr2 was observed in the epithelium, transition zone and fibers. By E13.5, the intensity of staining for Fgfr2 was reduced in these regions of the Dlg-1 deficient lenses. Thus, loss of Dlg-1 in the lens impairs Fgfr signaling and leads to altered levels of Fgfrs, suggesting that Dlg-1 is a modulator of Fgfr signaling pathway at the level of the receptors and that Dlg-1 regulates fiber cell differentiation through its role in PCP.

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

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

Primary cultures of embryonic chick lens cells as a model system to study lens gap junctions and fiber cell differentiation

A major limitation in lens gap junction research has been the lack of experimentally tractable ex vivo systems to study the formation and regulation of fiber-type gap junctions. Although immortalized lens-derived cell lines are amenable to both gene transfection and siRNA-mediated knockdown, to our knowledge none are capable of undergoing appreciable epithelial-to-fiber differentiation. Lens central epithelial explants have the converse limitation. A key advance in the field was the development of a primary embryonic chick lens cell culture system by Drs. Sue Menko and Ross Johnson. Unlike central epithelial explants, these cultures also include cells from the peripheral (preequatorial and equatorial) epithelium, which is the most physiologically relevant population for the study of fiber-type gap junction formation. We have modified the Menko/Johnson system and refer to our cultures as dissociated cell-derived monolayer cultures (DCDMLs). We culture DCDMLs without serum to mimic the avascular lens environment and on laminin, the major matrix component of the lens capsule. Here, I review the features of the DCDML system and how we have used it to study lens gap junctions and fiber cell differentiation. Our results demonstrate the power of DCDMLs to generate new findings germane to the mammalian lens and how these cultures can be exploited to conduct experiments that would be impossible, prohibitively expensive and/or difficult to interpret using transgenic animals in vivo.

Genetic epistasis between heparan sulfate and FGF-Ras signaling controls lens development

Vertebrate lens development depends on a complex network of signaling molecules to coordinate cell proliferation, migration and differentiation. In this study, we have investigated the role of heparan sulfate in lens specific signaling by generating a conditional ablation of heparan sulfate modification genes, Ndst1 and Ndst2. In this mutant, N-sulfation of heparan sulfate was disrupted after the lens induction stage, resulting in reduced lens cell proliferation, increased cell death and defective lens fiber differentiation in later lens development. The loss of Ndst function also prevented the assembly of Fgf/Fgfr complexes on the lens cell surface and disrupted ERK signaling within the lens. We further demonstrated that Ndst mutation completely inhibited the FGF1 and Fgf3 overexpression phenotypes, but Kras reactivation was sufficient to reverse the Ndst deficient lens differentiation defect. The epistatic relationship between Ndst and FGF-Ras signaling demonstrates that FGF signaling is the predominant signaling pathway controlled by Ndst in lens development.

Activation of unfolded protein response in transgenic mouse lenses

PURPOSE

Overloading of unfolded or misfolded proteins in the endoplasmic reticulum (ER) can cause ER stress and activate the unfolded protein response (UPR) in the cell. The authors tested whether transgene overexpression in the mouse lens would activate the UPR.

METHODS

Transgenic mice expressing proteins that either enter the ER secretory pathway or are synthesized in cytosol were selected. Activation of the UPR was assessed by determining the expression levels of the ER chaperone protein BiP, the spliced form of X-box binding protein-1 (Xbp-1) mRNA, and the transcription factor CHOP. Changes in the ubiquitin-proteasome system in the mouse lens were detected by ubiquitin immunofluorescence.

RESULTS

BiP expression was upregulated in the fiber cells of transgenic mouse lenses expressing platelet-derived growth factor-A (PDGF-A), dominant-negative fibroblast growth factor receptor (DN-FGFR), or DN-Sprouty2 (DN-Spy2). BiP upregulation occurred around embryonic day 16.5, primarily in the fiber cells adjacent to the organelle free zone. Fiber cell differentiation was disrupted in the PDGF-A and DN-Spry2 lenses, whereas the fiber cells were degenerating in the DN-FGFR lens. High levels of UPR activation and ubiquitin-labeled protein aggregates were found in the DN-FGFR lens, indicating inefficient disposal of unfolded/misfolded proteins in the fiber cells.

CONCLUSIONS

This study implies that overexpression of some transgenes in the lens can induce ER or overall cell stress in fiber cells, resulting in the activation of UPR signaling pathways. Therefore, investigators should assess the levels of UPR activation when they analyze the downstream effects of transgene expression in the lens.

Lens fiber cell differentiation and denucleation are disrupted through expression of the N-terminal nuclear receptor box of NCOA6 and result in p53-dependent and p53-independent apoptosis

Nuclear receptor coactivator 6 (NCOA6) is a multifunctional protein implicated in embryonic development, cell survival, and homeostasis. An 81-amino acid fragment, dnNCOA6, containing the N-terminal nuclear receptor box (LXXLL motif) of NCOA6, acts as a dominant-negative (dn) inhibitor of NCOA6. Here, we expressed dnNCOA6 in postmitotic transgenic mouse lens fiber cells. The transgenic lenses showed reduced growth; a wide spectrum of lens fiber cell differentiation defects, including reduced expression of gamma-crystallins; and cataract formation. Those lens fiber cells entered an alternate proapoptotic pathway, and the denucleation (karyolysis) process was stalled. Activation of caspase-3 at embryonic day (E)13.5 was followed by double-strand breaks (DSBs) formation monitored via a biomarker, gamma-H2AX. Intense terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) signals were found at E16.5. Thus, a window of approximately 72 h between these events suggested prolonged though incomplete apoptosis in the lens fiber cell compartment that preserved nuclei in its cells. Genetic experiments showed that the apoptotic-like processes in the transgenic lens were both p53-dependent and p53-independent. Lens-specific deletion of Ncoa6 also resulted in disrupted lens fiber cell differentiation. Our data demonstrate a cell-autonomous role of Ncoa6 in lens fiber cell differentiation and suggest novel insights into the process of lens fiber cell denucleation and apoptosis.

Kynurenine inhibits fibroblast growth factor 2-mediated expression of crystallins and MIP26 in lens epithelial cells

Fibroblast growth factor-2 (FGF2)-mediated signaling plays an important role in fiber cell differentiation in eye lens. We had previously shown that kynurenine (KYN) produced from the overexpression of indoleamine 2,3-dioxygenase (IDO) causes defects in the differentiation of fiber cells, induces fiber cell apoptosis and cataract formation in the mouse lens, and leads to cell cycle arrest in cultured mouse lens epithelial cells (mLEC). In this study, we demonstrate that exogenous KYN reduces FGF2-mediated expression of alpha-, beta-, and gamma-crystallin and MIP26 in mLEC. We show that endogenously produced KYN in mLEC of IDO transgenic animals causes similar defects in FGF2-induced protein expression and that a competitive inhibitor of IDO prevents such defects. Our data also show that KYN inhibits FGF2-induced Akt and ERK1/2 phosphorylation in mLEC, which are required for crystallin and MIP26 expression in the lens. KYN does not inhibit FGF2 binding to cells but inhibit phosphorylation of FGFR1in mLEC. Together our data suggest that KYN might inhibit FGF2-mediated fiber cell differentiation by preventing expression of crystallins and MIP26. Our studies provide a novel mechanism by which KYN can exert deleterious effects in cells.

Cell-autonomous requirements for Dlg-1 for lens epithelial cell structure and fiber cell morphogenesis

Cell polarity and adhesion are thought to be key determinants in organismal development. In Drosophila, discs large (dlg) has emerged as an important regulator of epithelial cell proliferation, adhesion, and polarity. Herein, we investigated the role of the mouse homolog of dlg (Dlg-1) in the development of the mouse ocular lens. Tissue-specific ablation of Dlg-1 throughout the lens early in lens development led to an expansion and disorganization of the epithelium that correlated with changes in the distribution of adhesion and polarity factors. In the fiber cells, differentiation defects were observed. These included alterations in cell structure and the disposition of cell adhesion/cytoskeletal factors, delay in denucleation, and reduced levels of alpha-catenin, pERK1/2, and MIP26. These fiber cell defects were recapitulated when Dlg-1 was disrupted only in fiber cells. These results suggest that Dlg-1 acts in a cell autonomous manner to regulate epithelial cell structure and fiber cell differentiation.

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

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

Indoleamine 2,3-dioxygenase overexpression causes kynurenine-modification of proteins, fiber cell apoptosis and cataract formation in the mouse lens

Indoleamine 2,3-dioxygenase (IDO) is the first enzyme in the kynurenine pathway. The kynurenines formed in this pathway chemically modify proteins and cause apoptosis in cells. Evidence suggests that kynurenines and their protein modifications are involved in cataract formation, but this has yet to be directly demonstrated. We generated transgenic (Tg) mouse lines that overexpress human IDO in the lens. Homozygous Tg (homTg) lenses had higher IDO immunoreactivity, approximately 4.5 times greater IDO mRNA, and approximately 8 times higher IDO activity compared to lenses from hemizygous Tg (hemTg) animals. The kynurenine content was threefold higher in homTg than in hemTg but was not detected in wild-type (Wt) lenses. Kynurenine modifications were approximately 2.6 times greater in homTg than in hemTg or Wt. HomTg lenses had vacuoles in the epithelium and cortical fiber cells. Kynurenine modifications coincided with apoptosis in the secondary fiber cells of homTg lenses. Caspase-3 and caspase-9 activities were markedly higher in homTg than in hemTg and Wt. The glutathione content was approximately 36% lower in homTg compared to hemTg and Wt lenses. HomTg animals also developed bilateral cataracts within 3 months of birth. Together these data demonstrate that IDO-mediated production of kynurenines results in defects in fiber cell differentiation and their apoptosis and suggest that IDO activity is kept low in the lens to prevent deleterious effects by kynurenines.

Growth factors involved in aqueous humour-induced lens cell proliferation

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

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

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

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

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

Fibroblast growth factor receptor signaling is essential for lens fiber cell differentiation

The vertebrate lens provides an excellent model to study the mechanisms that regulate terminal differentiation. Although fibroblast growth factors (FGFs) are thought to be important for lens cell differentiation, it is unclear which FGF receptors mediate these processes during different stages of lens development. Deletion of three FGF receptors (Fgfr1-3) early in lens development demonstrated that expression of only a single allele of Fgfr2 or Fgfr3 was sufficient for grossly normal lens development, while mice possessing only a single Fgfr1 allele developed cataracts and microphthalmia. Profound defects were observed in lenses lacking all three Fgfrs. These included lack of fiber cell elongation, abnormal proliferation in prospective lens fiber cells, reduced expression of the cell cycle inhibitors p27(kip1) and p57(kip2), increased apoptosis and aberrant or reduced expression of Prox1, Pax6, c-Maf, E-cadherin and alpha-, beta- and gamma-crystallins. Therefore, while signaling by FGF receptors is essential for lens fiber differentiation, different FGF receptors function redundantly.

Generation of transparency and cellular organization in lens explants

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

Fgf19 is required for zebrafish lens and retina development

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

Genetic and epigenetic mechanisms of gene regulation during lens development

Recent studies demonstrated a number of links between chromatin structure, gene expression, extracellular signaling and cellular differentiation during lens development. Lens progenitor cells originate from a pool of common progenitor cells, the pre-placodal region (PPR) which is formed from a combination of extracellular signaling between the neural plate, naïve ectoderm and mesendoderm. A specific commitment to the lens program over alternate choices such as the formation of olfactory epithelium or the anterior pituitary is manifested by the formation of a thickened surface ectoderm, the lens placode. Mouse lens progenitor cells are characterized by the expression of a complement of lens lineage-specific transcription factors including Pax6, Six3 and Sox2, controlled by FGF and BMP signaling, followed later by c-Maf, Mab21like1, Prox1 and FoxE3. Proliferation of lens progenitors together with their morphogenetic movements results in the formation of the lens vesicle. This transient structure, comprised of lens precursor cells, is polarized with its anterior cells retaining their epithelial morphology and proliferative capacity, whereas the posterior lens precursor cells initiate terminal differentiation forming the primary lens fibers. Lens differentiation is marked by expression and accumulation of crystallins and other structural proteins. The transcriptional control of crystallin genes is characterized by the reiterative use of transcription factors required for the establishment of lens precursors in combination with more ubiquitously expressed factors (e.g. AP-1, AP-2alpha, CREB and USF) and recruitment of histone acetyltransferases (HATs) CBP and p300, and chromatin remodeling complexes SWI/SNF and ISWI. These studies have poised the study of lens development at the forefront of efforts to understand the connections between development, cell signaling, gene transcription and chromatin remodeling.

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

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

An essential role for FGF receptor signaling in lens development

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

Attenuation of retinal vascular development and neovascularization in transgenic mice over-expressing thrombospondin-1 in the lens

Thrombospondin-1 (TSP1) is an endogenous inhibitor of angiogenesis and induces endothelial cell (EC) apoptosis. To study the role TSP1 plays during vascular development and neovascularization, we assessed the effects of ectopic TSP1 expression in the lens on retinal vascularization in transgenic mice. The TSP1 over-expressing mice showed abnormalities in the development of retinal vasculature. There was a dramatic decrease in the density of superficial and deep vascular plexuses of the retina in transgenic mice. The retinal vessels in TSP1 transgenic mice also appeared nonuniform and abnormal in maturation. We detected an increase in the number of EC undergoing apoptosis, which was compensated, in part, by an increase in cell proliferation in retinal vasculature of TSP1 transgenic mice. The TSP1 transgenic mice also exhibited increased levels of vessel obliteration and a limited preretinal neovascularization during oxygen-induced ischemic retinopathy (OIR). Our results indicate increased expression of TSP1 attenuates normal retinal vascularization and preretinal neovascularization during OIR. Therefore, modulation of TSP1 expression may provide an effective mechanism for regulation of ocular angiogenesis.

Apoptosis in lens development and pathology

The ocular lens is a distinct system to study cell death for the following reasons. First, during animal development, the ocular lens is crafted into its unique shape. The crafting processes include cell proliferation, cell migration, and apoptosis. Moreover, the lens epithelial cells differentiate into lens fiber cells through a process, which utilizes the same regulators as those in apoptosis at multiple signaling steps. In addition, introduction of exogenous wild-type or mutant genes or knock-out of the endogenous genes leads to apoptosis of the lens epithelial cells followed by absence of the ocular lens or formation of abnormal lens. Finally, both in vitro and in vivo studies have shown that treatment of adult lens with stress factors induces apoptosis of lens epithelial cells, which is followed by cataractogenesis. The present review summarizes the current knowledge on apoptosis in the ocular lens with emphasis on its role in lens development and pathology.

Structural and immunocytochemical alterations in eye lens fiber cells from Cx46 and Cx50 knockout mice

In the current study we describe the changes of overall organization of lens fiber cells in connexin 46 (Cx46) and connexin 50 (Cx50) knockout mice. Morphometric analyses and the application of immunocytochemical techniques revealed that in Cx46 knockout lens (Cx46 -/-), where Cx50 is expressed alone, the postnatal differentiation of secondary fiber cells proceeds faster and is characterized by an increased number of smaller fiber cells. Conversely, in Cx50 knockout mice (Cx50 -/-), the lenticular mass is considerably reduced and characterized by a small number of fiber cells added during the postnatal period. The process of terminal differentiation was impaired and generated larger fiber cells still possessing cytoplasmic organelles. Freeze-fracture and fracture labeling revealed that the junctional assembly, packing organization and topographic interactions between connexons and MP26 differed when Cx46 and Cx50 were co-assembled in the wild-type or expressed separately in the two distinct knockout phenotypes. Filipin cytochemistry provided indirect evidence that Cx46 and Cx50 expressed alone are recruited into different lipid environments. Our results represent the structural proof that interaction of connexins and MP26 contributes to the overall organization of the fiber cells.

Ras signaling is essential for lens cell proliferation and lens growth during development

The vertebrate ocular lens is a simple and continuously growing tissue. Growth factor-mediated receptor tyrosine kinases (RTKs) are believed to be required for lens cell proliferation, differentiation and survival. The signaling pathways downstream of the RTKs remain to be elucidated. Here, we demonstrate the important role of Ras in lens development by expressing a dominant-negative form of Ras (dn-Ras) in the lens of transgenic mice. We show that lens in the transgenic mice was smaller and lens growth was severely inhibited as compared to the wild-type lens. However, the lens shape, polarity and transparency appeared normal in the transgenic mice. Further analysis showed that cell proliferation is inhibited in the dn-Ras lens. For example, the percentage of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells in epithelial layer was about 2- to 3-fold lower in the transgenic lens than in the wild-type lens, implying that Ras activity is required for normal cell proliferation during lens development. We also found a small number of apoptotic cells in both epithelial and fiber compartment of the transgenic lens, suggesting that Ras also plays a role in cell survival. Interestingly, although there was a delay in primary fiber cell differentiation, secondary fiber cell differentiation was not significantly affected in the transgenic mice. For example, the expression of beta- and gamma-crystallins, the marker proteins for fiber differentiation, was not changed in the transgenic mice. Biochemical analysis indicated that ERK activity, but not Akt activity, was significantly reduced in the dn-Ras transgenic lenses. Overall, our data imply that the RTK-Ras-ERK signaling pathway is essential for cell proliferation and, to a lesser extent, for cell survival, but not for crystallin gene expression during fiber differentiation. Thus, some of the fiber differentiation processes are likely mediated by RTK-dependent but Ras-independent pathways.

Involvement of cytoskelatal proteins and growth factor receptors during development of the human eye

The spatial and temporal distribution of nestin, cytokeratins (CKs), vimentin, glial fibrillary acidic protein (GFAP), neurofilaments (NFs), beta-tubulin as well as fibroblast growth factor receptors (FGFRs) and platelet-derived growth factor receptor beta (PDGF-Rbeta) were investigated in the developing human eye in eight conceptuses of 5-9 postovulatory weeks using immunostaining. Nestin was found in the neuroglial precursors and the radial glial fibres of the optic nerve. In the pigmented retina, nestin was present only in the 5th week, while at later stages (6-9th week), co-expression of CKs and vimentin was seen. Nestin, CKs, vimentin, and GFAP were observed in the precursors to various cell types in the neural retina. Additionally, their expression was also apparent in the lens epithelium, showing its gradual fading following the lens fibre elongation. They appeared in the mesenchymal cells of the cornea, the choroid, the sclera, and the corpus vitreum, too. In the corneal epithelium, co-expression of nestin and CKs was detected. NFs and beta-tubulin were confined to the differentiating retinal neuroblasts. Growth factor receptors were seen in the retina, the lens epithelium while less intensely in the lens fibres, the corneal epithelium, and the mesenchymal cells. During the early eye development (5-9th week), IFs expressing normal pattern of distribution as well as acting in concert might contribute to the normal developmental processes occurring in certain parts of the human eye. Additionally, NFs and beta-tubulin seem to have an important role in the retinal ganglion cell differentiation, while FGFRs and PDGF-Rbeta may regulate the cell proliferation, differentiation, and survival in various parts of the developing eye.

FGF19-FGFR4 signaling elaborates lens induction with the FGF8-L-Maf cascade in the chick embryo

The fibroblast growth factor (FGF) family is known to be involved in vertebrate eye development. However, distinct roles of individual FGF members during eye development remain largely elusive. Here, we show a detailed expression pattern of Fgf19 in chick lens development. Fgf19 expression initiated in the forebrain, and then became restricted to the distal portion of the optic vesicle abutting the future lens placode, where FGF receptor 4 (Fgfr4), a receptor for FGF19, was expressed. Fgf8, a positive regulator for L-Maf, was expressed in a portion of the optic vesicle. To examine the role of FGF19 signaling during early eye development, Fgf19 was misexpressed near the presumptive lens ectoderm; however, no alteration in the expression of lens marker genes was observed. Conversely, a secreted form of FGFR4 was misexpressed to inhibit an FGF19 signal, resulting in the induction of L-Maf expression. To further define the relationship between L-Maf and Fgf19, L-Maf misexpression was performed, resulting in ectopic induction of Fgf19 expression by Hamburger and Hamilton's stage 12/13. Furthermore, misexpression of Fgf8 induced Fgf19 expression in addition to L-Maf. These results suggest that FGF19-FGFR4 signaling plays a role in early lens development in collaboration with FGF8 signaling and L-Maf transcriptional system.

Growth factor regulation of lens development

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

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

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

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

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

Lens major intrinsic protein (MIP)/aquaporin 0 expression in rat lens epithelia explants requires fibroblast growth factor-induced ERK and JNK signaling

Lens major intrinsic protein (MIP), exclusive to the vertebrate lens, otherwise known as MIP26 and Aquaporin 0, is abundantly expressed as a lens fiber membrane protein. Although relatively less efficient compared with other aquaporins, MIP is suggested to function as a water channel, as an adhesion molecule, and is required for lens transparency. Because MIP is specifically expressed in lens fiber cells, we investigated in this study the activation of Mip expression after triggering differentiation of rat lens epithelia explants by fibroblast growth factor (FGF)-2. Here, we show that Mip expression in the lens cells is regulated by FGF-2. Using Real time PCR we demonstrate that endogenous Mip levels in the explants were up-regulated upon FGF-2 stimulation, in a concentration-dependent manner. Up-regulation of Mip at the transcriptional level was simultaneous with the activation of the FGF down-stream signaling components, ERK1/2 and JNK. Specific inhibitors, UO126 for ERK1/2 and SP600125 for JNK, abrogated Mip expression in response to FGF-2 in the explants. This inhibition pattern was recapitulated in reporter assays for transfection of the rat lens epithelia explants, driven by the Mip promoter (-1648/+44). Our studies show that ERK1/2 and JNK signaling pathways are required for Mip expression in lens epithelia explants induced to differentiate by FGF-2.

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

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

cAMP-induced differentiation of human neuronal progenitor cells is mediated by nuclear fibroblast growth factor receptor-1 (FGFR1)

Activation of cAMP signaling pathway and its transcriptional factor cyclic AMP response element binding protein (CREB) and coactivator are key determinants of neuronal differentiation and plasticity. We show that nuclear fibroblast growth factor receptor-1 (FGFR1) mediates cAMP-induced neuronal differentiation and regulates CREB and CREB binding protein (CBP) function in alpha-internexin-expressing human neuronal progenitor cells (HNPC). In proliferating HNPC, FGFR1 was associated with the cytoplasm and plasma membrane. Treatment with dB-cAMP induced nuclear accumulation of FGFR1 and caused neuronal differentiation, accompanied by outgrowth of neurites expressing MAP2 and neuron-specific neurofilament-L protein and enolase. HNPC transfected with nuclear/cytoplasmic FGFR1 or non-membrane FGFR1(SP-/NLS), engineered to accumulate exclusively in the cell nucleus, underwent neuronal differentiation in the absence of cAMP stimulation. In contrast, FGFR1/R4, with highly hydrophobic transmembrane domain of FGFR4, was membrane associated, did not enter the nucleus and failed to induce neuronal differentiation. Transfection of tyrosine kinase-deleted dominant negative receptor mutants, cytoplasmic/nuclear FGFR1(TK-) or nuclear FGFR1(SP-/NLS)(TK-), prevented cAMP-induced neurite outgrowth. Nuclear FGFR1 localized in speckle-like domains rich in phosphorylated histone 3 and splicing factors, regions known for active RNA transcription and processing, and activated the neurofilament-L gene promoter. FGFR1(SP-/NLS) transactivated CRE, up-regulated phosphorylation and transcriptional activity of CREB and stimulated the activity of CBP several-fold. Thus, cAMP-induced nuclear accumulation of FGFR1 provides a signal that triggers molecular events leading to neuronal differentiation.

Bmp signaling is required for development of primary lens fiber cells

We have investigated the role of Bmp signaling in development of the mouse lens using three experimental strategies. First, we have shown that the Bmp ligand inhibitor noggin can suppress the differentiation of primary lens fiber cells in explant culture. Second, we have expressed a dominant-negative form of the type 1 Bmp family receptor Alk6 (Bmpr1b – Mouse Genome Informatics) in the lens in transgenic mice and shown that an inhibition of primary fiber cell differentiation can be detected at E13.5. Interestingly, the observed inhibition of primary fiber cell development was asymmetrical and appeared only on the nasal side of the lens in the ventral half. Expression of the inhibitory form of Alk6 was driven either by the αA-cystallin promoter or the ectoderm enhancer from the Pax6 gene in two different transgenes. These expression units drive transgene expression in distinct patterns that overlap in the equatorial cells of the lens vesicle at E12.5. Despite the distinctions between the transgenes, they caused primary fiber cell differentiation defects that were essentially identical, which implied that the equatorial lens vesicle cells were responding to Bmp signals in permitting primary fiber cells to develop. Importantly, E12.5 equatorial lens vesicle cells showed cell-surface immunoreactivity for bone-morphogenetic protein receptor type 2 and nuclear immunoreactivity for the active, phosphorylated form of the Bmp responsive Smads. This indicated that these cells had the machinery for Bmp signaling and were responding to Bmp signals. We conclude that Bmp signaling is required for primary lens fiber cell differentiation and, given the asymmetry of the differentiation inhibition, that distinct differentiation stimuli may be active in different quadrants of the eye.

Regulated lens regeneration from isolated pigmented epithelial cells of newt iris in culture in response to FGF2/4

When a lens is removed from the newt eye, a new lens is regenerated from the pigmented epithelial cells of the dorsal iris, whereas the ventral iris never shows such an ability. It is important to clarify the nature of signaling molecules which act directly on the iris cells to accomplish lens regeneration from the iris and also to gain insight into the mechanism of dorso-ventral difference of the regeneration potential. To examine the effects of exogenous factors, we established an in vitro culture of reaggregates made from dissociated pigmented epithelial cells of dorsal or ventral halves of newt iris. Foci of depigmented cells appeared within the cell reaggregates, regardless of their origins, when the cell reaggregates were cultured with FGF2 or FGF4. In contrast, only the depigmented cells in the dorsal iris cell reaggregates underwent extensive proliferation and developed a lens with the synthesis of lens-specific crystallins, recapitulating the normal lens regeneration. On the other hand, neither FGF8, FGF10, EGF, VEGF, nor IGF promoted lens development from iris cell reaggregates. Consistent with the FGF-specific action, FGFR-specific inhibitor SU5402 suppressed the lens development from the cultured cell reaggregates. These results demonstrated that FGF2 or FGF4 is essential for the in vitro lens regeneration from the pigmented cells of the dorsal iris. In addition, these findings indicated that unequal competence in the dorsal and ventral iris to FGF2/4 contributes to the difference in lens forming ability between them.

Deregulated cell cycle control in lens epithelial cells by expression of inhibitors of tumor suppressor function

Previous studies have shown that cell cycle proteins such as retinoblastoma protein (pRB) are essential for cell cycle withdrawal in differentiating lens cells. However, little is known about which factors are critical for cell cycle control in the lens epithelial cells. Here we use the K14 promoter to direct expression of E6 and E7, oncogenes from human papillomavirus type 16, which are known to bind and inactivate p53 and pRB, as molecular tools to study cell cycle regulation in the lens epithelium of transgenic mice. Expression of either gene resulted in increased proliferation and apoptosis, and in the case of E6, a unique epithelial phenotype characterized by multilayering and intercellular vacuoles was observed. Lenses from mice expressing E7 mutants, which are defective in inactivating pRB proteins, were normal and the lens phenotype in the E6 mice was p53-independent. Thus, cell proliferation in the lens epithelium is controlled by multiple factors including, but not necessarily limited to, the pRB family.

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.

Novel fibroblast growth factor 2 transcripts are expressed in mouse embryos

We have discovered two new exons in the mouse fibroblast growth factor 2 (FGF-2 or bFGF) gene that can be alternatively spliced to the second coding exon of the gene. The newly identified exons 1b and 1c are located at, respectively, approximately 19 and 32 kb downstream of the canonical exon 1a. Using RT-PCR analysis, mRNAs containing exon 1c and canonical exons 2 and 3 were identified in embryonic limb, placenta, face, carcass and ocular tissues. A 3.7-kb transcript present in placenta and embryonic limb hybridizes with an exon 1c-derived probe in Northern blot analysis. Alternative splicing of exon 1c to exon 2 creates a transcript for which the predicted alternative FGF-2 (altFGF-2) polypeptide contains a novel N-terminal domain. Our data indicate that in mouse embryos multiple novel mRNA variants are transcribed from the FGF-2 locus using alternative splicing. These data suggest that proteins arising from these alternative transcripts may play a role in mouse embryogenesis.

Molecular cloning, developmental expression, and hormonal regulation of zebrafish (Danio rerio) beta crystallin B1, a member of the superfamily of beta crystallin proteins

The cDNA sequence of beta crystallin B1 was determined from zebrafish (Danio rerio) and compared to the corresponding genes of bovine, rat, chicken, human, and Xenopus. Multispecies comparison of superfamily diversity demonstrated beta crystallin B1 homology between zebrafish, bovine, chicken, and rat, but large distances to beta crystallin B2 and B3. Zebrafish cDNA has a size of 943 nucleotides and encodes a polypeptide of 233 amino acids. Zebrafish beta crystallin B1 shares 71.30, 75.86, and 71.00% similarities with bovine, chicken, and rat beta crystallin B1, respectively. Northern blot analysis revealed a single 0.9-kb beta crystallin B1 transcript which was expressed and progressively increased in the first 20 h of zebrafish embryogenesis. Whole-mount in situ hybridization revealed that the beta crystallin B1 transcript was only specifically expressed in the lens region of the eye. A starvation experiment revealed no variation in mRNA levels after 14 and 21 days. An experiment in which hormone was injected showed that the beta crystallin B1 transcript first increased 24 h after the injection of insulin-like growth factor I, insulin-like growth factor II, or growth hormone, then decreased 48 h after injection. The beta crystallin B1 transcript continuously increased after insulin was injected. Taken together, our results identify the early specific expression of beta crystallin B1 within the lens. Despite small differences, these results indicate that both the structure of the beta crystallin B1 protein and its involvement with regulation by growth factors appear to have been remarkably conserved.

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.

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.

Secreted FGFR3, but not FGFR1, inhibits lens fiber differentiation

The vertebrate lens has a distinct polarity with cuboidal epithelial cells on the anterior side and differentiated fiber cells on the posterior side. It has been proposed that the anterior-posterior polarity of the lens is imposed by factors present in the ocular media surrounding the lens (aqueous and vitreous humor). The differentiation factors have been hypothesized to be members of the fibroblast growth factor (FGF) family. Though FGFs have been shown to be sufficient for induction of lens differentiation both in vivo and in vitro, they have not been demonstrated to be necessary for endogenous initiation of fiber cell differentiation. To test this possibility, we have generated transgenic mice with ocular expression of secreted self-dimerizing versions of FGFR1 (FR1) and FGFR3 (FR3). Expression of FR3, but not FR1, leads to an expansion of proliferating epithelial cells from the anterior to the posterior side of the lens due to a delay in the initiation of fiber cell differentiation. This delay is most apparent postnatally and correlates with appropriate changes in expression of marker genes including p57(KIP2), Maf and Prox1. Phosphorylation of Erk1 and Erk2 was reduced in the lenses of FR3 mice compared with nontransgenic mice. Though differentiation was delayed in FR3 mice, the lens epithelial cells still retained their intrinsic ability to respond to FGF stimulation. Based on these results we propose that the initiation of lens fiber cell differentiation in mice requires FGF receptor signaling and that one of the lens differentiation signals in the vitreous humor is a ligand for FR3, and is therefore likely to be an FGF or FGF-like factor.

Novel nuclear signaling pathway mediates activation of fibroblast growth factor-2 gene by type 1 and type 2 angiotensin II receptors

In bovine adrenal medullary cells synergistically acting type 1 and type 2 angiotensin II (AII) receptors activate the fibroblast growth factor-2 (FGF-2) gene through a unique AII-responsive promoter element. Both the type 1 and type 2 AII receptors and the downstream cyclic adenosine 1',3'-monophosphate- and protein kinase C-dependent signaling pathways activate the FGF-2 promoter through a novel signal-transducing mechanism. This mechanism, which we have named integrative nuclear FGF receptor-1 signaling, involves the nuclear translocation of FGF receptor-1 and its subsequent transactivation of the AII-responsive element in the FGF-2 promoter.

A role for endogenous TGFalpha and associated signaling pathways in the differentiation of lens fiber cells

TGFalpha is hypothesized to be an endogenous regulator of lens fiber terminal differentiation. With immunofluorescence, TGFalpha was localized to differentiating cells in the lens epithelium and superficial fiber cell mass of the adult chicken. A similar pattern of localization was also noted when differentiating epithelial cells were cultured. Immunoneutralization of endogenous TGFalpha inhibited the accumulation of filensin, a unique intermediate filament protein subunit restricted to developing vertebrate lens fibers. ELISA assays quantified the effects of TGFalpha on filensin expression. Surprisingly, inhibition of the TGFalpha receptors' tyrosine kinase activity with nanomolar concentrations of PD153035 increased the accumulation of differentiated characteristics in the presence or absence of ligand. Morphologically, PD153035-treated cells grew as aggregated masses and spread less well onto the substrate. Accompanying these morphologic changes was a complete inhibition of cell division. Post-receptor signaling events were examined with cAMP assays and Western blotting. TGFalpha did not affect cAMP levels while isoproterenol, an additional mediator of lens cell differentiation, caused significant increases in cAMP levels. Activation of ERK2 via dual phosphorylation was noted in response to TGFalpha but not isoproterenol. PD153035 reduced, but did not eliminate, ERK2 phosphorylation in response to TGFalpha. Phosphorylation of the CREB transcription factor was also observed in response to TGFalpha or isoproterenol. These data indicate that endogenous ligands can influence the expression of differentiated characteristics in cultured chick lens cells. A focus of multiple signaling pathways affecting filensin expression is the CREB transcription factor. While increased ERK2 activation may be involved in stimulating cell division, lower levels of persistent ERK2 activation promotes differentiation thus indicating some form of signaling pathway compartmentalization. This could also mean that additional receptors for TGFalpha not inhibited by PD153035 are responsible for promoting differentiation in chick lens cells.