Lens-specific expression of PDGF-A alters lens growth and development

Tonic ErbB signaling underlies TGFβ-induced activation of ERK and is required for lens cell epithelial to myofibroblast transition

Fibrosis is a major, but incompletely understood, component of many diseases. The most common vision-disrupting complication of cataract surgery involves differentiation of residual lens cells into myofibroblasts. In serum-free primary cultures of lens epithelial cells (DCDMLs), inhibitors of either ERK or of ErbB signaling prevent TGFβ from upregulating both early (fibronectin) and late (αSMA) markers of myofibroblast differentiation. TGFβ stimulates ERK in DCDMLs within 1.5 h. Kinase inhibitors of ErbBs, but not of several other growth factor receptors in lens cells, reduce phospho ERK to below basal levels in the absence or presence of TGFβ. This effect is attributable to constitutive ErbB activity playing a major role in regulating the basal levels pERK. Additional studies support a model in which TGFβ-generated reactive oxygen species serve to indirectly amplify ERK signaling downstream of tonically active ErbBs to mediate myofibroblast differentiation. ERK activity is in turn essential for expression of ErbB1 and ErbB2, major inducers of ERK signaling. By mechanistically linking TGFβ, ErbB, and ERK signaling to myofibroblast differentiation, our data elucidate a new role for ErbBs in fibrosis and reveal a novel mode by which TGFβ directs lens cell fate.

Heparan sulfate proteoglycans (HSPGs) of the ocular lens

Heparan sulfate proteoglycans (HSPGs) reside in most cells; on their surface, in the pericellular milieu and/or extracellular matrix. In the eye, HSPGs can orchestrate the activity of key signalling molecules found in the ocular environment that promote its development and homeostasis. To date, our understanding of the specific roles played by individual HSPG family members, and the heterogeneity of their associated sulfated HS chains, is in its infancy. The crystalline lens is a relatively simple and well characterised ocular tissue that provides an ideal stage to showcase and model the expression and unique roles of individual HSPGs. Individual HSPG core proteins are differentially localised to eye tissues in a temporal and spatial developmental- and cell-type specific manner, and their loss or functional disruption results in unique phenotypic outcomes for the lens, and other ocular tissues. More recent work has found that different HS sulfation enzymes are also presented in a cell- and tissue-specific manner, and that disruption of these different sulfation patterns affects specific HS-protein interactions. Not surprisingly, these sulfated HS chains have also been reported to be required for lens and eye development, with dysregulation of HS chain structure and function leading to pathogenesis and eye-related phenotypes. In the lens, HSPGs undergo significant and specific changes in expression and function that can drive pathology, or in some cases, promote tissue repair. As master signalling regulators, HSPGs may one day serve as valuable biomarkers, and even as putative targets for the development of novel therapeutics, not only for the eye but for many other systemic pathologies.

Insights into the biochemical and biophysical mechanisms mediating the longevity of the transparent optics of the eye lens

In the human eye, a transparent cornea and lens combine to form the "refracton" to focus images on the retina. This requires the refracton to have a high refractive index "n," mediated largely by extracellular collagen fibrils in the corneal stroma and the highly concentrated crystallin proteins in the cytoplasm of the lens fiber cells. Transparency is a result of short-range order in the spatial arrangement of corneal collagen fibrils and lens crystallins, generated in part by post-translational modifications (PTMs). However, while corneal collagen is remodeled continuously and replaced, lens crystallins are very long-lived and are not replaced and so accumulate PTMs over a lifetime. Eventually, a tipping point is reached when protein aggregation results in increased light scatter, inevitably leading to the iconic protein condensation-based disease, age-related cataract (ARC). Cataracts account for 50% of vision impairment worldwide, affecting far more people than other well-known protein aggregation-based diseases. However, because accumulation of crystallin PTMs begins before birth and long before ARC presents, we postulate that the lens protein PTMs contribute to a "cataractogenic load" that not only increases with age but also has protective effects on optical function by stabilizing lens crystallins until a tipping point is reached. In this review, we highlight decades of experimental findings that support the potential for PTMs to be protective during normal development. We hypothesize that ARC is preventable by protecting the biochemical and biophysical properties of lens proteins needed to maintain transparency, refraction, and optical function.

Lineage Contribution of PDGFRα-Expressing Cells in the Developing Mouse Eye

PDGFRα signaling is critically important in ocular development. Previous data on PDGFRα lacks an expression map with high spatial and temporal resolution and lineage information. In this study, we aim to present a detailed PDGFRα expression and lineage map from early embryogenesis to adulthood. PDGFRα-CreER; mT/mG reporter mice were analyzed. mEGFP-positive cells contributed to multiple ocular lineages in a spatiotemporally regulated manner. A dynamic PDGFRα expression was identified in corneal stromal cells, lens epithelial cells, lens fiber cells, and retinal astrocytes during the entire period of eye development, while PDGFRα expression in retinal astrocytes from E17.5 onwards and in Müller glial cells was identified within two weeks after birth. By revealing detailed characterization of gene expression and function, we present a comprehensive map of PDGFRα-expressing cells in the eye for a better understanding of PDGFRα signaling's role during eye development.

Lens-specific conditional knockout of tropomyosin 1 gene in mice causes abnormal fiber differentiation and lens opacity

Tropomyosin (Tpm) 1 and 2 are important in the epithelial mesenchymal transition of lens epithelial cells; however, the effect of Tpm1 depletion during aging remains obscure. We analyzed the age-related changes in the crystalline lens of Tpm1- conditional knockout mice (Tpm1-CKO). Floxed alleles of Tpm1 were conditionally deleted in the lens, using Pax6-cre transgenic mice. Lenses of embryonic day (ED) 14, postnatal 1-, 11-, and 48-week-old Tpm1-CKO and wild type mice were dissected to prepare paraffin sections, which subsequently underwent histological and immunohistochemical analysis. Tpm1 and α smooth muscle actin (αSMA) mRNA expression were assessed using RT-PCR. The homozygous Tpm1-CKO (Tpm1^-/-) lenses displayed a dramatic reduction in Tpm1 transcript, with no change to αSMA mRNA expression. Tpm1^-/- mice had small lenses with disorganized, vesiculated fiber cells, and loss of epithelial cells. The lenses of Tpm1^-/- mice had abnormal and disordered lens fiber cells with cortical and peri-nuclear liquefaction. Expression of filamentous-actin was reduced in the equator region of lenses derived from ED14, 1-, 11-, and 48-week-old Tpm1^-/- mice. Therefore, Tpm1 plays an integral role in mediating the integrity and fate of lens fiber differentiation and lens homeostasis during aging. Age-related Tpm1 dysregulation or deficiency may induce cataract formation.

Development, structure, and bioengineering of the human corneal stroma: A review of collagen-based implants

Bio-engineering technologies are currently used to produce biomimetic artificial corneas that should present structural, chemical, optical, and biomechanical properties close to the native tissue. These properties are mainly supported by the corneal stroma which accounts for 90% of corneal thickness and is mainly made of collagen type I. The stromal collagen fibrils are arranged in lamellae that have a plywood-like organization. The fibril diameter is between 25 and 35 nm and the interfibrillar space about 57 nm. The number of lamellae in the central stroma is estimated to be 300. In the anterior part, their size is 10-40 μm. They appear to be larger in the posterior part of the stroma with a size of 60-120 μm. Their thicknesses also vary from 0.2 to 2.5 μm. During development, the acellular corneal stroma, which features a complex pattern of organization, serves as a scaffold for mesenchymal cells that invade and further produce the cellular stroma. Several pathways including Bmp4, Wnt/β-catenin, Notch, retinoic acid, and TGF-β, in addition to EFTFs including the mastering gene Pax-6, are involved in corneal development. Besides, retinoic acid and TGF- β seem to have a crucial role in the neural crest cell migration in the stroma. Several technologies can be used to produce artificial stroma. Taking advantage of the liquid-crystal properties of acid-soluble collagen, it is possible to produce transparent stroma-like matrices with native-like collagen I fibrils and plywood-like organization, where epithelial cells can adhere and proliferate. Other approaches include the use of recombinant collagen, cross-linkers, vitrification, plastically compressed collagen or magnetically aligned collagen, providing interesting optical and mechanical properties. These technologies can be classified according to collagen type and origin, presence of telopeptides and native-like fibrils, structure, and transparency. Collagen matrices feature transparency >80% for the appropriate 500-μm thickness. Non-collagenous matrices made of biopolymers including gelatin, silk, or fish scale have been developed which feature interesting properties but are less biomimetic. These bioengineered matrices still need to be colonized by stromal cells to fully reproduce the native stroma.

Multiple cytokine analyses of aqueous humor from the patients with retinitis pigmentosa

PURPOSE

Cataracts are the most common eye complications of retinitis pigmentosa (RP). This study aimed to investigate the cytokine profiles of the aqueous humor of RP with cataracts.

METHODS

The aqueous humor was collected from RP eyes with cataract (RP group, n = 20) and age-related cataract eyes (ARC group, n = 20) during cataract surgery. The levels of 37 mediators were measured with multiplex fluorescent bead-based immunoassay and compared across groups. The correlation among chemokines, growth factors, and cytokines was analyzed with Spearman's rank correlation coefficient.

RESULTS

Twelve cytokines (IL-1α, IL-1β, IL-4, IL-10, TNF-α, IFN-γ, EGF, GM-CSF, PDGF-AB/BB, TGF-α, BMP-9, and E-selection) were below the limit of detection, and the detection rate of IL-6 was significantly higher in RP group than in the ARC group (P < 0.01). Compared with those in the control group, the aqueous humor levels of monocyte chemoattractant protein-1 (MCP-1), interleukin-(IL-)8, interferon gamma-induced protein (IP)-10, hepatocyte growth factor (HGF), platelet-derived growth factor AA (PDGF-AA), matrix metalloproteinase-2 (MMP-2), MMP3, MMP-7, MMP-8, plasminogen activator inhibitor-1 (PAI-1), and thrombospondin-2 (TSP-2) in the RP group increased significantly (P < 0.01). A lower level of BMP-4 in the aqueous humor was observed in the RP patients than in the controls (P < 0.05).

CONCLUSIONS

Significantly increased levels of PDGF-AA, MMP2, MMP3, MMP-7, MMP-8, PAI-1, and TSP-2 and lower levels of BMP-4 were found in the aqueous humor of RP patients. This result indicates a disturbance of the extracellular matrix (ECM) and cytokines in RP patients and suggests a possible role of these cytokines in the pathogenesis of capsular contraction syndrome (CCS) in RP patients.

Lens differentiation is controlled by the balance between PDGF and FGF signaling

How multiple receptor tyrosine kinases coordinate cell fate determination is yet to be elucidated. We show here that the receptor for platelet-derived growth factor (PDGF) signaling recruits the p85 subunit of Phosphoinositide 3-kinase (PI3K) to regulate mammalian lens development. Activation of PI3K signaling not only prevents B-cell lymphoma 2 (BCL2)-Associated X (Bax)- and BCL2 Antagonist/Killer (Bak)-mediated apoptosis but also promotes Notch signaling to prevent premature cell differentiation. Reducing PI3K activity destabilizes the Notch intracellular domain, while the constitutive activation of Notch reverses the PI3K deficiency phenotype. In contrast, fibroblast growth factor receptors (FGFRs) recruit Fibroblast Growth Factor Receptor Substrate 2 (Frs2) and Rous sarcoma oncogene (Src) Homology Phosphatase 2 (Shp2) to activate Mitogen-Activated Protein Kinase (MAPK) signaling, which induces the Notch ligand Jagged 1 (Jag1) and promotes cell differentiation. Inactivation of Shp2 restored the proper timing of differentiation in the p85 mutant lens, demonstrating the antagonistic interaction between FGF-induced MAPK and PDGF-induced PI3K signaling. By selective activation of PI3K and MAPK, PDGF and FGF cooperate with and oppose each other to balance progenitor cell maintenance and differentiation.

A central aim in understanding cell signaling is to decode the cellular logic that underlies the functional specificity of growth factors. Although these factors are known to activate a common set of intracellular pathways, they nevertheless play specific roles in development and physiology. Using lens development in mice as a model, we show that fibroblast growth factor (FGF) and platelet-derived growth factor (PDGF) antagonize each other through their intrinsic biases toward distinct downstream targets. While FGF primarily induces the Ras–Mitogen-Activated Protein Kinase (MAPK) axis to promote lens cell differentiation, PDGF preferentially stimulates Phosphoinositide 3-kinase (PI3K) to enhance Notch signaling, which is necessary for maintaining the lens progenitor cell pool. By revealing the intricate interactions between PDGF, FGF, and Notch, we present a paradigm for how signaling crosstalk enables balanced growth and differentiation in multicellular organisms.

Presenilin gene function and Notch signaling feedback regulation in the developing mouse lens

Presenilins (Psen1 and Psen2 in mice) are polytopic transmembrane proteins that act in the γ-secretase complex to make intra-membrane cleavages of their substrates, including the well-studied Notch receptors. Such processing releases the Notch intracellular domain, allowing it to physically relocate from the cell membrane to the nucleus where it acts in a transcriptional activating complex to regulate downstream genes in the signal-receiving cell. Previous studies of Notch pathway mutants for Jagged1, Notch2, and Rbpj demonstrated that canonical signaling is a necessary component of normal mouse lens development. However, the central role of Psens within the γ-secretase complex has never been explored in any developing eye tissue or cell type. By directly comparing Psen single and double mutant phenotypes during mouse lens development, we found a stronger requirement for Psen1, although both genes are needed for progenitor cell growth and to prevent apoptosis. We also uncovered a novel genetic interaction between Psen1 and Jagged1. By quantifying protein and mRNA levels of key Notch pathway genes in Psen1/2 or Jagged1 mutant lenses, we identified multiple points in the overall signaling cascade where feedback regulation can occur. Our data are consistent with the loss of particular genes indirectly influencing the transcription level of another. However, we conclude that regulating Notch2 protein levels is particularly important during normal signaling, supporting the importance of post-translational regulatory mechanisms in this tissue.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Aqueous humour-induced lens epithelial cell proliferation requires FGF-signalling

The eye lens grows by systematic proliferation of its epithelial cells and their differentiation into fibre cells. The anterior aqueous humour regulates lens epithelial cell proliferation whereas posteriorly, the vitreous stimulates lens fibre differentiation. Vitreous-derived members of the fibroblast growth factor (FGF) family induce fibre differentiation, with added support for FGFs as putative regulators of aqueous-induced lens cell proliferation. To further characterize this, given FGFs' known affinity for proteoglycans, we compared the effect of proteoglycan sulphation in growth factor- and aqueous-induced lens cell proliferation. Disruption of proteoglycan sulphation in lens cells specifically impacted on aqueous- and FGF-induced MAPK/ERK1/2-signalling, but not on that induced by other mitogens such as PDGF; however, cell proliferation was reduced in all treatment groups, regardless of the mitogen. Overall, by disrupting proteoglycan activity, we further highlight the significant role of FGFs in aqueous-induced ERK1/2 phosphorylation leading to lens cell proliferation.

Synergistic interaction between the fibroblast growth factor and bone morphogenetic protein signaling pathways in lens cells

Relatively little is known about how receptor tyrosine kinase ligands can positively cooperate with BMP signaling. Primary cultures of lens cells were used to reveal an unprecedented type of cross-talk between the canonical FGF and BMP signaling pathways that regulates lens cell differentiation and intercellular coupling.

Fibroblast growth factors (FGFs) play a central role in two processes essential for lens transparency—fiber cell differentiation and gap junction–mediated intercellular communication (GJIC). Using serum-free primary cultures of chick lens epithelial cells (DCDMLs), we investigated how the FGF and bone morphogenetic protein (BMP) signaling pathways positively cooperate to regulate lens development and function. We found that culturing DCDMLs for 6 d with the BMP blocker noggin inhibits the canonical FGF-to-ERK pathway upstream of FRS2 activation and also prevents FGF from stimulating FRS2- and ERK-independent gene expression, indicating that BMP signaling is required at the level of FGF receptors. Other experiments revealed a second type of BMP/FGF interaction by which FGF promotes expression of BMP target genes as well as of BMP4. Together these studies reveal a novel mode of cooperation between the FGF and BMP pathways in which BMP keeps lens cells in an optimally FGF-responsive state and, reciprocally, FGF enhances BMP-mediated gene expression. This interaction provides a mechanistic explanation for why disruption of either FGF or BMP signaling in the lens leads to defects in lens development and function.

Characterization of platelet-derived growth factor-A expression in mouse tissues using a lacZ knock-in approach

Expression of the platelet-derived growth factor A-chain gene (Pdgfa) occurs widely in the developing mouse, where it is mainly localized to various epithelial and neuronal structures. Until now, in situ mRNA hybridization (ISH) has been the only reliable method to identify Pdgfa expression in tissue sections or whole mount preparations. Validated protocols for in situ detection of PDGF-A protein by immunohistochemistry is lacking. In particular, this has hampered understanding of Pdgfa expression pattern in adult tissues, where ISH is technically challenging. Here, we report a gene targeted mouse Pdgfa allele, Pdgfa^ex4COIN, which is a combined conditional knockout and reporter allele. Cre-mediated inversion of the COIN cassette inactivates Pdgfa coding while simultaneously activating a beta-galactosidase (lacZ) reporter under endogenous Pdgfa transcription control. The generated Pdgfa^{ex4COIN-INV-lacZ} allele can next be used to identify cells carrying a Pdgfa null allele, as well as to map endogenous Pdgfa expression. We evaluated the Pdgfa^{ex4COIN-INV-lacZ} allele as a reporter for endogenous Pdgfa expression patterns in mouse embryos and adults. We conclude that the expression pattern of Pdgfa^{ex4COIN-INV-lacZ} recapitulates known expression patterns of Pdgfa. We also report on novel embryonic and adult Pdgfa expression patterns in the mouse and discuss their implications for Pdgfa physiology.

The lens equator: a platform for molecular machinery that regulates the switch from cell proliferation to differentiation in the vertebrate lens

The vertebrate lens is a transparent, spheroidal tissue, located in the anterior region of the eye that focuses visual images on the retina. During development, surface ectoderm associated with the neural retina invaginates to form the lens vesicle. Cells in the posterior half of the lens vesicle differentiate into primary lens fiber cells, which form the lens fiber core, while cells in the anterior half maintain a proliferative state as a monolayer lens epithelium. After formation of the primary fiber core, lens epithelial cells start to differentiate into lens fiber cells at the interface between the lens epithelium and the primary lens fiber core, which is called the equator. Differentiating lens fiber cells elongate and cover the old lens fiber core, resulting in growth of the lens during development. Thus, lens fiber differentiation is spatially regulated and the equator functions as a platform that regulates the switch from cell proliferation to cell differentiation. Since the 1970s, the mechanism underlying lens fiber cell differentiation has been intensively studied, and several regulatory factors that regulate lens fiber cell differentiation have been identified. In this review, we focus on the lens equator, where these regulatory factors crosstalk and cooperate to regulate lens fiber differentiation. Normally, lens epithelial cells must pass through the equator to start lens fiber differentiation. However, there are reports that when the lens epithelium structure is collapsed, lens fiber cell differentiation occurs without passing the equator. We also discuss a possible mechanism that represses lens fiber cell differentiation in lens epithelium.

MAPK1 is required for establishing the pattern of cell proliferation and for cell survival during lens development

The mitogen-activated protein kinases (MAPKs; also known as ERKs) are key intracellular signaling molecules that are ubiquitously expressed in tissues and were assumed to be functionally equivalent. Here, we use the mouse lens as a model system to investigate whether MAPK1 plays a specific role during development. MAPK3 is known to be dispensable for lens development. We demonstrate that, although MAPK1 is uniformly expressed in the lens epithelium, its deletion significantly reduces cell proliferation in the peripheral region, an area referred to as the lens germinative zone in which most active cell division occurs during normal lens development. By contrast, cell proliferation in the central region is minimally affected by MAPK1 deletion. Cell cycle regulators, including cyclin D1 and survivin, are downregulated in the germinative zone of the MAPK1-deficient lens. Interestingly, loss of MAPK1 subsequently induces upregulation of phosphorylated MAPK3 (pMAPK3) levels in the lens epithelium; however, this increase in pMAPK3 is not sufficient to restore cell proliferation in the germinative zone. Additionally, MAPK1 plays an essential role in epithelial cell survival but is dispensable for fiber cell differentiation during lens development. Our data indicate that MAPK1/3 control cell proliferation in the lens epithelium in a spatially defined manner; MAPK1 plays a unique role in establishing the highly mitotic zone in the peripheral region, whereas the two MAPKs share a redundant role in controlling cell proliferation in the central region of the lens epithelium.

Expression of pro- and anti-angiogenic factors during the formation of the periocular vasculature and development of the avian cornea

BACKGROUND

During embryonic development, endothelial precursor cells (angioblasts) migrate relatively long distances to form the primary vascular plexus. The migratory behavior of angioblasts and localization of the primitive blood vessels is tightly regulated by pro-angiogenic and anti-angiogenic factors encountered in the embryonic environment. Despite the importance of corneal avascularity to proper vision, it is not known when avascularity is established in the developing cornea and how pro- and anti-angiogenic factors regulate this process.

RESULTS AND DISCUSSION

Using Tg(tie1:H2B:eYFP) transgenic quail embryos to visualize fluorescently labeled angioblasts, we show that the presumptive cornea remains avascular despite the invasion of cells from the periocular region where migratory angioblasts reside and form the primary vasculature. Semiquantitative reverse transcriptase polymerase chain reaction analysis and spatiotemporal examination of gene expression revealed that pro- and anti-angiogenic factors were expressed in patterns indicating their potential roles in angioblast guidance.

CONCLUSIONS

Our findings show for the first time that chick corneal avascularity is established and maintained during development as the periocular vasculature forms. We also identify potential candidate pro- and anti-angiogenic factors that may play crucial roles during vascular patterning in the anterior eye.

Frs2α enhances fibroblast growth factor-mediated survival and differentiation in lens development

Most growth factor receptor tyrosine kinases (RTKs) signal through similar intracellular pathways, but they often have divergent biological effects. Therefore, elucidating the mechanism of channeling the intracellular effect of RTK stimulation to facilitate specific biological responses represents a fundamental biological challenge. Lens epithelial cells express numerous RTKs with the ability to initiate the phosphorylation (activation) of Erk1/2 and PI3-K/Akt signaling. However, only Fgfr stimulation leads to lens fiber cell differentiation in the developing mammalian embryo. Additionally, within the lens, only Fgfrs activate the signal transduction molecule Frs2α. Loss of Frs2α in the lens significantly increases apoptosis and decreases phosphorylation of both Erk1/2 and Akt. Also, Frs2α deficiency decreases the expression of several proteins characteristic of lens fiber cell differentiation, including Prox1, p57(KIP2), aquaporin 0 and β-crystallins. Although not normally expressed in the lens, the RTK TrkC phosphorylates Frs2α in response to binding the ligand NT3. Transgenic lens epithelial cells expressing both TrkC and NT3 exhibit several features characteristic of lens fiber cells. These include elongation, increased Erk1/2 and Akt phosphorylation, and the expression of β-crystallins. All these characteristics of NT3-TrkC transgenic lens epithelial cells depend on Frs2α. Therefore, tyrosine phosphorylation of Frs2α mediates Fgfr-dependent lens cell survival and provides a mechanistic basis for the unique fiber-differentiating capacity of Fgfs on mammalian lens epithelial cells.

New insights into the mechanism of lens development using zebra fish

On the basis of recent advances in molecular biology, genetics, and live-embryo imaging, direct comparisons between zebra fish and human lens development are being made. The zebra fish has numerous experimental advantages for investigation of fundamental biomedical problems that are often best studied in the lens. The physical characteristics of visible light can account for the highly coordinated cell differentiation during formation of a beautifully transparent, refractile, symmetric optical element, the biological lens. The accessibility of the zebra fish lens for direct investigation during rapid development will result in new knowledge about basic functional mechanisms of epithelia-mesenchymal transitions, cell fate, cell-matrix interactions, cytoskeletal interactions, cytoplasmic crowding, membrane transport, cell adhesion, cell signaling, and metabolic specialization. The lens is well known as a model for characterization of cell and molecular aging. We review the recent advances in understanding vertebrate lens development conducted with zebra fish.

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.

PDGF induced microRNA alterations in cancer cells

Platelet derived growth factor (PDGF) regulates gene transcription by binding to specific receptors. PDGF plays a critical role in oncogenesis in brain and other tumors, regulates angiogenesis, and remodels the stroma in physiologic conditions. Here, we show by using microRNA (miR) arrays that PDGFs regulate the expression and function of miRs in glioblastoma and ovarian cancer cells. The two PDGF ligands AA and BB affect expression of several miRs in ligand-specific manner; the most robust changes consisting of let-7d repression by PDGF-AA and miR-146b induction by PDGF-BB. Induction of miR-146b by PDGF-BB is modulated via MAPK-dependent induction of c-fos. We demonstrate that PDGF regulates expression of some of its known targets (e.g. cyclin D1) through miR alterations and identify the epidermal growth factor receptor (EGFR) as a new PDGF-BB target. We show that its expression and function are repressed by PDGF-induced miR-146b and that mir-146b and EGFR correlate inversely in human glioblastomas. We propose that PDGF-regulated gene transcription involves alterations in non-coding RNAs and provide evidence for a miR-dependent feedback mechanism balancing growth factor receptor signaling in cancer cells.

Time-specific blockade of PDGFR with Imatinib (Glivec®) causes cataract and disruption of lens fiber cells in neonatal mice

This study aimed at investigating the response of lens epithelial cells in postnatal mice to Imatinib (Glivec®, a potent inhibitor of platelet-derived growth factor receptor (PDGFR)) treatment. Mouse eyes were sampled 10 days after administration of Imatinib (0.5 mg·g(-1)·day(-1)) for 3 days, at either 7, 14, or 21 days postpartum. Structural changes of lens were revealed by routine H.E. staining. Levels of proliferation and apoptosis were revealed by BrdU incorporation and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, respectively, and immunofluorescent staining with anti-PDGFRα antibody was carried out on the sections of eyeball. PDGFRα and p-PDGFRαprotein levels were evaluated by Western blot. Our results indicated that administration of Imatinib led to blockade of PDGFR signaling. Formation of cataracts was found only in those mice where treatment started from 7 days postpartum (P7), but was not observed in those samples from P14 nor P21. Fiber cells were disorganized in cataract lens core as observed histologically, and migration of epithelial cells was also inhibited. No apoptosis was detected with the TUNEL method. Our results indicated blockade of PDGFR at the neonatal stage (P7) would lead to cataracts and lens fiber cells disorganization, suggesting that PDGFR signaling plays a time-specific and crucial role in the postnatal development of lens in the mouse, and also may provide a new approach to produce a congenital cataract animal model.

Growth factor signaling in vitreous humor-induced lens fiber differentiation

PURPOSE. Although some of the factors and signaling pathways that are involved in induction of fiber differentiation have been defined, such as FGF-mediated MAPK/ERK and PI3-K/Akt signaling, the factors in the vitreous that regulate this differentiation process in vivo have yet to be identified. The purpose of this study was to better understand the role of growth factors in vitreous that regulate this process by further characterizing the signaling pathways involved in lens fiber differentiation. METHODS. Rat lens epithelial explants were used to compare the ability of vitreous, IGF-1, PDGF-A, EGF, and FGF-2 to stimulate the phosphorylation of ERK1/2 and Akt leading to fiber differentiation, in the presence or absence of selective receptor tyrosine kinase (RTK) inhibitors. RESULTS. Similar to vitreous, FGF induced a sustained ERK1/2 signaling profile, unlike IGF, PDGF, and EGF, which induced a more transient (shorter) activation of ERK1/2. For Akt activation, IGF was the only factor that induced a profile similar to vitreous. IGF, PDGF, and EGF potentiated the effects of a low dose of FGF on lens fiber differentiation by extending the duration of ERK1/2 phosphorylation. In the presence of selective RTK inhibitors, although the sustained vitreous-induced ERK1/2 signaling profile and subsequent fiber differentiation was perturbed, the results also showed that, although prolonged ERK1/2 phosphorylation was necessary, it was not sufficient for fiber differentiation to proceed. CONCLUSIONS. These results are consistent with FGF's being the key growth factor involved in vitreous-induced signaling leading to lens fiber differentiation; however, they also indicate that other vitreal growth factors such as IGF may be involved in fine-tuning ERK1/2- and Akt-phosphorylation to the level that is necessary for initiation and/or maintenance of lens fiber differentiation in vivo.

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.

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.

Homeodomain protein Pitx3 maintains the mitotic activity of lens epithelial cells

Pitx3 is a bicoid like homeobox transcription factor of which deficiency in mice is linked with the aphakia phenotype. Mutation in human PITX3 gene is associated with autosomal dominant cataract with variable anterior segment mesenchymal dysgenesis. However, the molecular events causing the morphological changes in aphakia remains unknown. In this study we investigated the behaviour of GFP tagged Pitx3 null embryonic stem cells in chimeric lens, as well as the molecular features of the Pitx3-deficient lens of homozygous Pitx3 knockout mice. We show that the lack of colonisation of Pitx3-deficient ES cell derivatives in Pitx3 wild-type<-->Pitx3 null chimeric lens was due to the depletion of the epithelial cells in lens epithelium manifested by aberrant cell cycle exit and precocious onset of fibre cell differentiation of the Pitx3 null cells at the lens vesicle stage. This was demonstrated by the early activation of the cell cycle inhibitors p27Kip1 and p57Kip2, and the expression of beta-and gamma-crystallins. These defects are at least partially attributed to the loss of FoxE3 and misexpression of Prox1 in the lens vesicle epithelial cells. Thus, Pitx3 is essential to maintain lens epithelial phenotype and prevent inappropriate fibre cell differentiation during lens development.

Role of platelet-derived growth factors in physiology and medicine

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) have served as prototypes for growth factor and receptor tyrosine kinase function for more than 25 years. Studies of PDGFs and PDGFRs in animal development have revealed roles for PDGFR-alpha signaling in gastrulation and in the development of the cranial and cardiac neural crest, gonads, lung, intestine, skin, CNS, and skeleton. Similarly, roles for PDGFR-beta signaling have been established in blood vessel formation and early hematopoiesis. PDGF signaling is implicated in a range of diseases. Autocrine activation of PDGF signaling pathways is involved in certain gliomas, sarcomas, and leukemias. Paracrine PDGF signaling is commonly observed in epithelial cancers, where it triggers stromal recruitment and may be involved in epithelial-mesenchymal transition, thereby affecting tumor growth, angiogenesis, invasion, and metastasis. PDGFs drive pathological mesenchymal responses in vascular disorders such as atherosclerosis, restenosis, pulmonary hypertension, and retinal diseases, as well as in fibrotic diseases, including pulmonary fibrosis, liver cirrhosis, scleroderma, glomerulosclerosis, and cardiac fibrosis. We review basic aspects of the PDGF ligands and receptors, their developmental and pathological functions, principles of their pharmacological inhibition, and results using PDGF pathway-inhibitory or stimulatory drugs in preclinical and clinical contexts.

Differential requirement for beta-catenin in epithelial and fiber cells during lens development

Recent studies implicate Wnt/beta-catenin signaling in lens differentiation (Stump, R. J., et al., 2003. A role for Wnt/beta-catenin signaling in lens epithelial differentiation. Dev Biol;259:48-61). Beta-catenin is a component of adherens junctions and functions as a transcriptional activator in canonical Wnt signaling. We investigated the effects of Cre/LoxP-mediated deletion of beta-catenin during lens development using two Cre lines that specifically deleted beta-catenin in whole lens or only in differentiated fibers, from E13.5. We found that beta-catenin was required in lens epithelium and during early fiber differentiation but appeared to be redundant in differentiated fiber cells. Complete loss of beta-catenin resulted in an abnormal and deficient epithelial layer with loss of E-cadherin and Pax6 expression as well as abnormal expression of c-Maf and p57(kip2) but not Prox1. There was also disrupted fiber cell differentiation, characterized by poor cell elongation, decreased beta-crystallin expression, epithelial cell cycle arrest at G(1)-S transition and premature cell cycle exit. Despite cell cycle arrest there was no induction of apoptosis. Mutant fiber cells displayed altered apical-basal polarity as evidenced by altered distribution of the tight junction protein, ZO1, disruption of apical actin filaments and abnormal deposition of extracellular matrix, resulting in a deficient lens capsule. Loss of beta-catenin also affected the formation of adhesion junctions as evidenced by dissociation of N-cadherin and F-actin localization in differentiating fiber cells. However, loss of beta-catenin from terminally differentiating fibers had no apparent effects on adhesion junctions between adjacent embryonic fibers. These data indicate that beta-catenin plays distinct functions during lens fiber differentiation and is involved in both Wnt signaling and adhesion-related mechanisms that regulate lens epithelium and early fiber differentiation.

Notch signaling regulates growth and differentiation in the mammalian lens

The Notch signal transduction pathway regulates the decision to proliferate versus differentiate. Although there are a myriad of mouse models for the Notch pathway, surprisingly little is known about how these genes regulate early eye development, particularly in the anterior lens. We employed both gain-of-function and loss-of-function approaches to determine the role of Notch signaling in lens development. Here we analyzed mice containing conditional deletion of the Notch effector Rbpj or overexpression of the activated Notch1 intracellular domain during lens formation. We demonstrate distinct functions for Notch signaling in progenitor cell growth, fiber cell differentiation and maintenance of the transition zone. In particular, Notch signaling controls the timing of primary fiber cell differentiation and is essential for secondary fiber cell differentiation. Either gain or loss of Notch signaling leads to formation of a dysgenic lens, which in loss-of-function mice undergoes a profound postnatal degeneration. Our data suggest both Cyclin D1 and Cyclin D2, and the p27(Kip1) cyclin-dependent kinase inhibitor act downstream of Notch signaling, and define multiple critical functions for this pathway during lens development.

Age-dependent control of lens growth by hypoxia

PURPOSE

The lens grows continuously throughout life, but the factors that influence the size of the adult lens are not known. Lens thickness is a significant risk factor for age-related cataract. It has been postulated that the hypoxic environment in the eye protects the lens from nuclear cataracts. The authors sought to determine whether the Po(2) in the eye regulates lens growth.

METHODS

Lens cell proliferation was determined by counting BrdU-labeled and total nuclei in the germinative zone in flatmounts of lens epithelia. Oxygen levels in the eye were altered by having rats breathe 11%, 21% (room air), or 60% oxygen. Oxygen levels in the vitreous were measured with a fiberoptic oxygen sensor.

RESULTS

The BrdU-labeling index in the germinative zone declined from approximately 3.5% at 1 month to less than 0.7% at 8 months. Raising oxygen levels in the eyes of 1-month-old animals did not alter the rate of lens cell proliferation. Elevating intraocular oxygen in animals older than 1 month increased proliferation to the more rapid rate seen at 1 month. Decreasing oxygen levels below their normally low level did not affect the BrdU-labeling index at any age. Chronic exposure to increased oxygen led to the production of more lens fiber cells and larger lenses.

CONCLUSIONS

Normal age-related decline in lens growth requires the low oxygen level normally present in the eye. Increases in lens cell number and mass may account for some of the increase in cataract risk caused by chronic exposure of the lens to elevated oxygen levels.

The Etiology of Steroid Cataract

Steroid-induced posterior subcapsular cataracts (PSCs) exhibit three main distinctive characteristics: (i) association only with steroids possessing glucocorticoid activity, (ii) involvement of aberrant migrating lens epithelial cells, and (iii) a central posterior location. The first characteristic suggests a key role for glucocorticoid receptor activation and subsequent changes to the transcription of specific genes. Glucocorticoid receptor activation is associated in many cell types with proliferation, suppressed differentiation, a reduced susceptibility to apoptosis, altered transmembrane transport, and enhancement of reactive oxygen species activity. Glucocorticoids may be capable of inducing changes to the transcription of genes in lens epithelial cells that are related to many of these cellular processes. This review examines the various mechanisms that have been proposed to account for the development of PSC in the context of recent DNA array studies. Additionally, given that the glucocorticoid receptor can also engender wide-ranging indirect activities, glucocorticoids could also indirectly affect the lens through the responses of other cells within the ocular compartment and/or through effects on cells at more remote locations. These indirect mechanisms, which, for example, could be mediated through alterations to the intraocular levels of growth factors that normally orchestrate lens development and maintain lens homeostasis, are also discussed. Although the mechanism of steroid cataract induction remains unknown, glucocorticoid-induced gene transcription events in lens epithelial cells, and also other intraocular or systemic cells, likely interact to generate steroid cataracts. Finally, although evidence for glucocorticoid-protein adduct formation in the lens is inconclusive, the generation of such adducts cannot yet be discounted as a contributing factor and must necessarily be retained in discussions of the etiology of steroid cataract.

Foxe view of lens development and disease

The recent identification of a mutation in Foxe3 that causes congenital primary aphakia in humans marks an important milestone. Congenital primary aphakia is a rare developmental disease in which the lens does not form. Previously, Foxe3 had been shown to play a crucial role in vertebrate lens formation and this gene is one of the earliest integrators of several signaling pathways that cooperate to form a lens. In this review, we highlight recent advances that have led to a better understanding of the developmental processes and gene regulatory networks involved in lens development and disease.

Unique vascular phenotypes following over-expression of individual VEGFA isoforms from the developing lens

Formation of a correctly organised vasculature and subsequently embryonic survival is critically dependent on the dosage and site-specific expression of VEGF. Murine VEGF exists in three common isoforms (viz. 120, 164 and 188 amino acids) having different organ specific distribution levels. Gene knock-in studies show that expression of any of the individual isoforms of VEGF extends survival until birth, although each is associated with distinct organ-specific abnormalities. Comparison of the effects of VEGF isoform expression is complicated by the general lethality of mis-expression, in addition to cumulative effects of adjacent tissues from the inappropriately patterned vasculature. Here we investigate the effects of over-expression of individual VEGFA isoforms from the lens-specific alphaA-Crystallin promoter and characterise their effects on the vessel morphology of the hyaloid and developing retinal vasculature. Since the hyaloid vasculature is an anatomically distinct, transient vasculature of the eye, comprising 3 cell types (endothelium, pericytes and macrophages) it is possible to more readily interpret the role of individual VEGF-A isoforms in vascular pattern formation in this model. The severity of the vascular phenotype, characterised by a hyperplastic hyaloid at E13.5 and subsequently retinal vascular patterning and ocular defects, is most severe in transgenics over-expressing the more diffusible forms of VEGFA (120 and 164), whereas in VEGFA(188) transgenics the hyaloid vascular defects partially resolve post-natally. The results of this study indicate that individual isoforms of VEGFA induce distinct vascular phenotypes in the eye during embryonic development and that their relative doses provide instructive cues for vascular patterning.

Cell cycle regulation in the developing lens

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

An essential role for FGF receptor signaling in lens development

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

Molecular events in early development of the ciliary body: a question of folding

Ciliary body morphogenesis is a complicated, multi-step process requiring coordinated changes in cell shape, flexure of epithelial sheets and dynamic shifts in mitotic rates. Very little is known of how these cellular events are triggered or regulated. This review summarises current models of ciliary body morphogenesis. The role of intraocular pressure as a driver of morphogenesis is re-evaluated in the light of new information. An update on the role of the lens in ciliary body morphogenesis is presented. In the second part of the review current gene expression data is related to ciliary body morphogenesis. In particular the role of Bmp4 and its downstream target genes are discussed, with novel gene expression patterns of Bmp4 and Tgfbeta1i4 being presented.

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.

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

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

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.

Insight into the physiological functions of PDGF through genetic studies in mice

Genetic analyses in mice have contributed significantly to the understanding of the physiological functions of platelet-derived growth factors (PDGFs) and their receptors. Phenotypic analyses of gene knockouts of PDGF-A, PDGF-B, PDGF alpha-receptors (PDGFRalpha) and beta-receptors (PDGFRbeta) have shown that these ligands and receptors play major roles during embryonic development. Conditional and subtle mutations in the same genes and analysis of chimeric mice have provided additional information about the roles of these genes in postnatal development. Transgenic over-expression studies have also demonstrated that PDGF ligands are capable of inducing pathological cell proliferation in a number of different organs. The present review summarizes these findings and discusses their implications for mammalian development and disease.

Platelet-derived growth factor D, tissue-specific expression in the eye, and a key role in control of lens epithelial cell proliferation

Platelet-derived growth factor D (PDGF-D), also known as Iris-expressed growth factor, is a member of the PDGF/vascular endothelial growth factor family. The expression of PDGF-D in the eye is tissue-specific. In the anterior segment, it is localized to iris and ciliary body, whereas in the retina, PDGF-D is restricted to the outer plexiform layer. PDGF-D is present in aqueous humor but is not detectable in mature lens or in mouse lens-derived alphaTN4-1 cells. However, it is expressed in rabbit lens-derived N/N1003A cells. N/N1003A cell-conditioned medium stimulates proliferation in rat lens explants, and this is blocked by immunodepletion of PDGF-D. Immunopurified PDGF-D also stimulates cell proliferation in rat lens explants and in NIH 3T3 cells. In organ culture of rat eye anterior segments, anti-PDGF-D strongly inhibits lens epithelial cell proliferation. This finding suggests a major in vivo role for PDGF-D in the mechanisms of coordinated growth of eye tissues. Intervention in the PDGF-D pathway in the eye, perhaps by antibody or blocking peptide, could be useful in the treatment of certain cataracts, including post-operative secondary cataract.

Expression of c-Fos and c-Jun in developing mouse lens

We carried out immunohistochemistry in embryonic eye-specific antibodies in the mouse to determine the spatial/temporal expression pattern of c-Fos and c-Jun proteins--the main components of the AP-1 transcription factor, in lens epithelial cells during mouse lens morphogenesis. c-Fos protein expression was detected in the equatorial epithelium at E14.5-P25, with a peak at P1-P8. c-Jun protein expression was detected in equatorial lens epithelial cells at E18.5-P8, with a peak at P1. During these intervals, the anterior epithelium lacked expression of c-Fos and c-Jun. They were not detected in an adult mouse lens. The expression pattern of the AP-1 transcription factor in equatorial epithelium may suggest its role in fiber differentiation.

Inhibition of Lens Fiber Cell Morphogenesis by Expression of a Mutant SV40 Large T Antigen That Binds CREB-binding Protein/p300 but Not pRb

Simian virus (SV) 40 large T antigen can both induce tumors and inhibit cellular differentiation. It is not clear whether these cellular changes are synonymous, sequential, or distinct responses to the protein. T antigen is known to bind to p53, to the retinoblastoma (Rb) family of tumor suppressor proteins, and to other cellular proteins such as p300 family members. To test whether SV40 large T antigen inhibits cellular differentiation in vivo in the absence of cell cycle induction, we generated transgenic mice that express in the lens a mutant version of the early region of SV40. This mutant, which we term E107KDelta, has a deletion that eliminates synthesis of small t antigen and a point mutation (E107K) that results in loss of the ability to bind to Rb family members. At embryonic day 15.5 (E15.5), the transgenic lenses show dramatic defects in lens fiber cell differentiation. The fiber cells become post-mitotic, but do not elongate properly. The cells show a dramatic reduction in expression of their beta- and gamma-crystallins. Because CBP and p300 are co-activators for crystallin gene expression, we assayed for interactions between E107KDelta and CBP/p300. Our studies demonstrate that cellular differentiation can be inhibited by SV40 large T antigen in the absence of pRb inactivation, and that interaction of large T antigen with CBP/p300 may be enhanced by a mutation that eliminates the binding to pRb.

Optical development in the foetal bovine lens

The refractive index variations along the optic axes of nine foetal bovine lenses were measured using an optic fibre reflectometer. The device measures refractive index directly, using the principle of Fresnel reflectance. These results show that lenses with a wet weight >0.7 g (corresponding to 4.5 months gestational age), have a refractive index profile which approximates a parabolic shape, akin to that found in post-natal and adult bovine lenses. In lenses of wet weight below 0.7 g, the refractive index distributions show irregularities. These findings suggest that the refractive index profile in the bovine lens starts to form into a parabolic distribution, as seen in adult lenses, about half-way through gestation. The trigger for this formation is not known.

Transgenic overexpression of platelet-derived growth factor-C in the mouse heart induces cardiac fibrosis, hypertrophy, and dilated cardiomyopathy

The platelet-derived growth factors are implicated in development of fibrotic reactions and disease in several organs. We have overexpressed platelet-derived growth factor-C in the heart using the alpha-myosin heavy chain promoter and created a transgenic mouse that exhibits cardiac fibrosis followed by hypertrophy with sex-dependent phenotypes. The transgenic mice developed several pathological changes including cardiac fibroblast proliferation and deposition of collagen, hypertrophy, vascular defects, and the presence of Anitschkow cells in the adult myocardium. Male mice developed a hypertrophic phenotype, whereas female mice were more severely affected and developed dilated cardiomyopathy, leading to heart failure and sudden death. The vascular defects initially included dilation of microvessels and vascular leakage. Subsequently, a marked loss of microvessels, formation of large vascular sac-like structures, and an increased density of smooth muscle-coated vessels were observed in the myocardium. In part, the observed vascular changes may be because of an up-regulation of vascular endothelial growth factor in cardiac fibroblasts of the transgenic hearts. This unique animal model reveals that a potent mitogen for cardiac fibroblasts result in an expansion of the interstitium that induce a secondary sex-dependent hypertrophic response in the cardiomyocytes.

Suppression of lens growth by alphaA-crystallin promoter-driven expression of diphtheria toxin results in disruption of retinal cell organization in zebrafish

In order to study lens-retina relationships during development, we cloned the zebrafish alphaA-crystallin cDNA and its promoter region. Using a 2.8-kb fragment of the zebrafish alphaA-crystallin promoter (z(alpha)Acry), we expressed the diphtheria toxin A fragment (DTA) in zebrafish embryos in a lens-specific manner. Injection of the z(alpha)Acry-DTA plasmid into eggs at the one-or two-cell stage resulted in the formation of small eyes, in which both lens and retina were reduced in size. In the DTA-expressing lenses, their fiber structure was disorganized, indicating that normal lens development had been abrogated. The neural retina also showed abnormal development, although this tissue did not express DTA. Lamination in the retina did not develop well, and molecular markers for the outer and inner plexiform layers were either abnormally expressed or absent. However, cell type-specific markers of ganglion and bipolar cells, as well as photoreceptors, were expressed in appropriate positions, indicating that initial differentiation of these retinal subpopulations occurred in the DTA-expressing embryos. Cell proliferation also proceeded normally in these embryos, although apoptosis was enhanced. These results suggest that the differentiated lens plays a critical role in the morphogenetic organization of retinal cells during eye development in zebrafish embryos.

Disruption of anterior segment development by TGF-β1 overexpression in the eyes of transgenic mice

Previous experiments showed that transgenic mice expressing a secreted self-activating transforming growth factor (TGF) -beta1 did not show a phenotype in the lens and cornea until postnatal day 21, when anterior subcapsular cataracts, sporadic thickening of the corneal stroma, and thinning of the corneal epithelium were noted (Srinivasan et al., 1998). To examine the effects of higher concentrations of TGF-beta1 on the lens and cornea, we constructed transgenic mice harboring the strong, lens-specific chicken betaB1-crystallin promoter driving an activated porcine TGF-beta1 gene. In contrast to the earlier study, the transgenic mice had microphthalmic eyes with closed eyelids. Already at embryonic day (E) 13.5, the future cornea of the transgenic mice was threefold thicker than that of wild-type littermates due to increased proliferation of corneal stromal mesenchyme cells. Staining of fibronectin and thrombospondin-1 was increased in periocular mesenchyme. At E17.5, the thickened transgenic corneal stroma was vascularized and densely populated by abundant star-shaped, neural cell adhesion molecule-positive cells of mesenchymal appearance surrounded by irregular swirls of collagen and extracellular matrix. The corneal endothelium, anterior chamber, and stroma of iris/ciliary body did not develop, and the transgenic cornea was opaque. Fibronectin, perlecan, and thrombospondin-1 were elevated, whereas type VI collagen decreased in the transgenic corneal stroma. Stromal mesenchyme cells expressed alpha-smooth muscle actin as did lens epithelial cells and cells of the retinal pigmented epithelium. By E17.5, lens fiber cells underwent apoptotic cell death that was followed by apoptosis of the entire anterior lens epithelium between E18.5 and birth. Posteriorly, the vitreous humor was essentially absent; however, the retina appeared relatively normal. Thus, excess TGF-beta1, a mitogen for embryonic corneal mesenchyme, severely disrupts corneal and lens differentiation. Our findings profoundly contrast with the mild eye phenotype observed with presumably lower levels of ectopic TGF-beta and illustrate the complexity of TGF-beta utilization and the importance of dose when assessing the effects of this growth factor.

Retinal ablation and altered lens differentiation induced by ocular overexpression of BMP7

The alphaA-crystallin promoter was used to target expression of bone morphogenetic protein 7 (BMP7) to lens fiber cells in transgenic mice. Surprisingly, lens-specific expression of BMP7 induced widespread apoptosis and rapid ablation of the neural retina in multiple families. Subsequent to retinal ablation, the lens bow region shifted posteriorly until lens epithelial cells completely enveloped the lens. Lens-specific expression of FGF3 was found to rescue the loss of fiber cell differentiation. Our results show that elevated BMP7 levels can induce rapid retinal degeneration accompanied by disruption of the endogenous ocular system for fiber cell induction.

Developmental roles of platelet-derived growth factors

Platelet-derived growth factor (PDGF) was originally identified in platelets and in serum as a mitogen for fibroblasts, smooth muscle cells (SMC) and glia cells in culture. PDGF has since expanded to a family of dimers of at least four gene products, whose biological actions are mediated through two receptor tyrosine kinases, PDGFRs. The present review summarizes and discusses the biological functions of PDGFs and PDGFRs in developmental processes, mainly as revealed through genetic analysis in mice. Such studies have demonstrated multiple critical roles of PDGFs and PDGFRs in embryonic and postnatal development. PDGFs seem to act upon specific populations of progenitor cells that give rise to several different cell types with distinct functions in a variety of developmental processes. Analogies are seen between the cell functions and the developmental processes controlled by PDGFs. This suggests that ancestral PDGF and PDGFR expression patterns and functions may have been iterated in related sets of morphogenetic processes in the course of evolution.