Pax-6 and lens-specific transcription of the chicken delta 1-crystallin gene

Multiomics Analysis Reveals Novel Genetic Determinants for Lens Differentiation, Structure, and Transparency

Recent advances in next-generation sequencing and data analysis have provided new gateways for identification of novel genome-wide genetic determinants governing tissue development and disease. These advances have revolutionized our understanding of cellular differentiation, homeostasis, and specialized function in multiple tissues. Bioinformatic and functional analysis of these genetic determinants and the pathways they regulate have provided a novel basis for the design of functional experiments to answer a wide range of long-sought biological questions. A well-characterized model for the application of these emerging technologies is the development and differentiation of the ocular lens and how individual pathways regulate lens morphogenesis, gene expression, transparency, and refraction. Recent applications of next-generation sequencing analysis on well-characterized chicken and mouse lens differentiation models using a variety of omics techniques including RNA-seq, ATAC-seq, whole-genome bisulfite sequencing (WGBS), chip-seq, and CUT&RUN have revealed a wide range of essential biological pathways and chromatin features governing lens structure and function. Multiomics integration of these data has established new gene functions and cellular processes essential for lens formation, homeostasis, and transparency including the identification of novel transcription control pathways, autophagy remodeling pathways, and signal transduction pathways, among others. This review summarizes recent omics technologies applied to the lens, methods for integrating multiomics data, and how these recent technologies have advanced our understanding ocular biology and function. The approach and analysis are relevant to identifying the features and functional requirements of more complex tissues and disease states.

Molecular signature for senile and complicated cataracts derived from analysis of sumoylation enzymes and their substrates in human cataract lenses

Sumoylation is one of the key regulatory mechanisms in eukaryotes. Our previous studies reveal that sumoylation plays indispensable roles during lens differentiation (Yan et al. 2010. Proc Natl Acad Sci USA. 107:21034-21039; Gong et al. 2014. Proc Natl Acad Sci USA. 111:5574-5579). Whether sumoylation is implicated in cataractogenesis, a disease largely derived from aging, remains elusive. In the present study, we have examined the changing patterns of the sumoylation ligases and de-sumoylation enzymes (SENPs) and their substrates including Pax6 and other proteins in cataractous lenses of different age groups from 50 to 90 years old. It is found that compared with normal lenses, sumoylation ligases 1 and 3, de-sumoylation enzymes SENP3/7/8, and p46 Pax6 are clearly increased. In contrast, Ubc9 is significantly decreased. Among different cataract patients from 50s to 70s, male patients express more sumoylation enzymes and p46 Pax6. Ubc9 and SENP6 display age-dependent increase. The p46 Pax6 displays age-dependent decrease in normal lens, remains relatively stable in senile cataracts but becomes di-sumoylated in complicated cataracts. In contrast, sumoylation of p32 Pax6 is observed in senile cataracts and increases its stability. Treatment of rat lenses with oxidative stress increases Pax6 expression without sumoylation but promotes apoptosis. Thus, our results show that the changing patterns in Ubc9, SENP6, and Pax6 levels can act as molecular markers for senile cataract and the di-sumoylated p46 Pax6 for complicated cataract. Together, our results reveal the presence of molecular signature for both senile and complicated cataracts. Moreover, our study indicates that sumoylation is implicated in control of aging and cataractogenesis.

Analysis of Non-Sumoylated and Sumoylated Isoforms of Pax-6, the Master Regulator for Eye and Brain Development in Ocular Cell Lines

PURPOSE

Pax-6 is a master regulator for eye and brain development. Previous studies including ours have shown that Pax-6 exists in 4 major isoforms. According to their sizes, they are named p48, p46, p43 and p32 with the corresponding molecular weight of 48, 46, 43 and 32 kd, respectively. While p48 and p46 is derived from alternative splicing, p32 Pax-6 is generated through an internal translation initiation site. As for 43 kd Pax-6, two resources have been reported. In bird, it was found that an alternative splicing can generate a p43 Pax-6. In human and mouse, we reported that the p43 kd Pax-6 is derived from sumoylation: addition of a 11 kd polypeptide SUMO1 into the p32 Pax-6 at the K91 residue. Whether other Pax-6 isoforms can be sumoylated or not remains to be explored.

METHODS

The 5 major ocular cell lines were cultured in Dulbecco's modified Eagle's medium (DMEM) containing fetal bovine serum (FBS) or rabbit serum (RBS) and 1% Penicillin- Streptomycin. The mRNA levels were analysed with qRT-PCR. The protein levels were determined with western blot analysis and quantitated with Image J.

RESULTS

Both non-sumoylated and sumoylated isoforms of Pax-6 exist in 6 major types of ocular cells among which five are lens epithelial cells, and one is retinal pigment epithelial cell. Our results revealed that the most abundant isoforms of Pax-6 are the p32 and p46 Pax-6. These two major isoforms can be sumoylated to generate p43 (mono-sumoylated p32 Pax-6), p57 and p68 Pax-6 (mono- and di-sumoylated p46 Pax-6). In addition, the splicing-generated p48 Pax-6 is also readily detected.

CONCLUSION

Our results for the first time, have determined the relative isoform abundance and also the sumoylation patterns of pax-6 in 6 major ocular cell lines.

Chicken GRIFIN: binding partners, developmental course of localization and activation of its lens-specific gene expression by L-Maf/Pax6

Tissue lectins appear to be involved in a broad range of physiological processes, as reflected for the members of the family of galectins by referring to them as adhesion/growth-regulatory effectors. In order to clarify the significance of galectin presence, key challenges are to define their binding partners and the profile of localization. Having identified the chicken galectin-related interfiber protein (C-GRIFIN) as lens-specific protein present in the main body of adult lens, we here report its interaction with lens proteins in ligand blotting. The assumption for pairing with α-, β- and δ-crystallins was ascertained by mass spectrometric detection of their presence in eluted fractions obtained by affinity chromatography. Biochemical and immunohistochemical monitoring revealed protein presence from about 3-day-old embryos onwards, mostly in the cytoplasm of elongated posterior cells, later in secondary lens fiber cells. On the level of gene expression, its promoter was activated by transcription factor L-Maf alone and together with Pax6 like a crystallin gene, substantiating C-GRIFIN's status as lens-specific galectin. Using this combined strategy for counterreceptor and expression profiling by bio- and histochemical methods including light, electron and fluorescence microscopy, respective monitoring in lens development can now be taken to the level of the complete galectin family.

A comprehensive spatial-temporal transcriptomic analysis of differentiating nascent mouse lens epithelial and fiber cells

Elucidation of both the molecular composition and organization of the ocular lens is a prerequisite to understand its development, function, pathology, regenerative capacity, as well as to model lens development and disease using in vitro differentiation of pluripotent stem cells. Lens is comprised of the anterior lens epithelium and posterior lens fibers, which form the bulk of the lens. Lens fibers differentiate from lens epithelial cells through cell cycle exit-coupled differentiation that includes cellular elongation, accumulation of crystallins, cytoskeleton and membrane remodeling, and degradation of organelles within the central region of the lens. Here, we profiled spatiotemporal expression dynamics of both mRNAs and non-coding RNAs from microdissected mouse nascent lens epithelium and lens fibers at four developmental time points (embryonic [E] day 14.5, E16.5, E18.5, and P0.5) by RNA-seq. During this critical time window, multiple complex biosynthetic and catabolic processes generate the molecular and structural foundation for lens transparency. Throughout this developmental window, 3544 and 3518 genes show consistently and significantly greater expression in the nascent lens epithelium and fibers, respectively. Comprehensive data analysis confirmed major roles of FGF-MAPK, Wnt/β-catenin, PI3K/AKT, TGF-β, and BMP signaling pathways and revealed significant novel contributions of mTOR, EIF2, EIF4, and p70S6K signaling in lens formation. Unbiased motif analysis within promoter regions of these genes with consistent expression changes between epithelium and fiber cells revealed an enrichment for both established (e.g. E2Fs, Etv5, Hsf4, c-Maf, MafG, MafK, N-Myc, and Pax6) transcription factors and a number of novel regulators of lens formation, such as Arntl2, Dmrta2, Stat5a, Stat5b, and Tulp3. In conclusion, the present RNA-seq data serves as a comprehensive reference resource for deciphering molecular principles of normal mammalian lens differentiation, mapping a full spectrum of signaling pathways and DNA-binding transcription factors operating in both lens compartments, and predicting novel pathways required to establish lens transparency.

Loss of Sip1 leads to migration defects and retention of ectodermal markers during lens development

SIP1 encodes a DNA-binding transcription factor that regulates multiple developmental processes, as highlighted by the pleiotropic defects observed in Mowat-Wilson syndrome, which results from mutations in this gene. Further, in adults, dysregulated SIP1 expression has been implicated in both cancer and fibrotic diseases, where it functionally links TGFβ signaling to the loss of epithelial cell characteristics and gene expression. In the ocular lens, an epithelial tissue important for vision, Sip1 is co-expressed with epithelial markers, such as E-cadherin, and is required for the complete separation of the lens vesicle from the head ectoderm during early ocular morphogenesis. However, the function of Sip1 after early lens morphogenesis is still unknown. Here, we conditionally deleted Sip1 from the developing mouse lens shortly after lens vesicle closure, leading to defects in coordinated fiber cell tip migration, defective suture formation, and cataract. Interestingly, RNA-Sequencing analysis on Sip1 knockout lenses identified 190 differentially expressed genes, all of which are distinct from previously described Sip1 target genes. Furthermore, 34% of the genes with increased expression in the Sip1 knockout lenses are normally downregulated as the lens transitions from the lens vesicle to early lens, while 49% of the genes with decreased expression in the Sip1 knockout lenses are normally upregulated during early lens development. Overall, these data imply that Sip1 plays a major role in reprogramming the lens vesicle away from a surface ectoderm cell fate towards that necessary for the development of a transparent lens and demonstrate that Sip1 regulates distinctly different sets of genes in different cellular contexts.

Ocular aldehyde dehydrogenases: protection against ultraviolet damage and maintenance of transparency for vision

Aldehyde dehydrogenase (ALDH) enzymes catalyze the NAD(P)(+)-dependent oxidation of a wide variety of endogenous and exogenous aldehydes to their corresponding acids. Some members of the ALDH superfamily of enzymes are abundantly expressed in the mammalian cornea and lens in a taxon-specific manner. Considered to be corneal and lens crystallins, they confer protective and transparent properties upon these ocular tissues. ALDH3A1 is highly expressed in the cornea of most mammals, with the exception of rabbit that expresses exclusively ALDH1A1 in the cornea. ALDH1A1 is present in both the cornea and lens of several animal species. As a result of their catalytic and non-catalytic functions, ALDH3A1 and ALDH1A1 proteins protect inner ocular tissues from ultraviolet radiation and reactive oxygen-induced damage. In addition, these corneal crystallins contribute to cellular transparency in corneal stromal keratocytes, supporting a structural role of these ALDH proteins. A putative regulatory function of ALDH3A1 on corneal cell proliferation has also been proposed. Finally, the three retinaldehyde dehydrogenases cooperatively mediate retinoic acid signaling during the eye development.

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.

Sumoylation activates the transcriptional activity of Pax-6, an important transcription factor for eye and brain development

Pax-6 is an evolutionarily conserved transcription factor regulating brain and eye development. Four Pax-6 isoforms have been reported previously. Although the longer Pax-6 isoforms (p46 and p48) bear two DNA-binding domains, the paired domain (PD) and the homeodomain (HD), the shorter Pax-6 isoform p32 contains only the HD for DNA binding. Although a third domain, the proline-, serine- and threonine-enriched activation (PST) domain, in the C termini of all Pax-6 isoforms mediates their transcriptional modulation via phosphorylation, how p32 Pax-6 could regulate target genes remains to be elucidated. In the present study, we show that sumoylation at K91 is required for p32 Pax-6 to bind to a HD-specific site and regulate expression of target genes. First, in vitro-synthesized p32 Pax-6 alone cannot bind the P3 sequence, which contains the HD recognition site, unless it is preincubated with nuclear extracts precleared by anti-Pax-6 but not by anti-small ubiquitin-related modifier 1 (anti-SUMO1) antibody. Second, in vitro-synthesized p32 Pax-6 can be sumoylated by SUMO1, and the sumoylated p32 Pax-6 then can bind to the P3 sequence. Third, Pax-6 and SUMO1 are colocalized in the embryonic optic and lens vesicles and can be coimmunoprecipitated. Finally, SUMO1-conjugated p32 Pax-6 exists in both the nucleus and cytoplasm, and sumoylation significantly enhances the DNA-binding ability of p32 Pax-6 and positively regulates gene expression. Together, our results demonstrate that sumoylation activates p32 Pax-6 in both DNA-binding and transcriptional activities. In addition, our studies demonstrate that p32 and p46 Pax-6 possess differential DNA-binding and regulatory activities.

The transcription factor D-Pax2 regulates crystallin production during eye development in Drosophila melanogaster

The generation of a functioning Drosophila eye requires the coordinated differentiation of multiple cell types and the morphogenesis of eye-specific structures. Here we show that D-Pax2 plays a significant role in lens development through regulation of the Crystallin gene and because Crystallin is also expressed in D-Pax2(+) cells in the external sensory organs. Loss of D-Pax2 function leads to loss of Crystallin expression in both eyes and bristles. A 2.3 kilobase (kb) upstream region of the Crystallin gene can drive GFP expression in the eye and is dependent on D-Pax2. In addition, D-Pax2 binds to an evolutionarily conserved site in this region that, by itself, is sufficient to drive GFP expression in the eye. However, mutation of this site does not greatly affect the regulatory region's function. The data indicate that D-Pax2 acts to promote lens development by controlling the production of the major protein component of the lens. Whether this control is direct or indirect remains unresolved.

The molecular basis of defective lens development in the Iberian mole

Background

Fossorial mammals face natural selection pressures that differ from those acting on surface dwelling animals, and these may lead to reduced visual system development. We have studied eye development in a species of true mole, the Iberian mole Talpa occidentalis, and present the molecular basis of abnormal lens development. This is the first embryological developmental study of the eyes of any fossorial mammal at the molecular level.

Results

Lens fibre differentiation is not completed in the Iberian mole. Although eye development starts normally (similar to other model species), defects are seen after closure of the lens vesicle. PAX6 is not down-regulated in developing lens fibre nuclei, as it is in other species, and there is ectopic expression of FOXE3, a putative downstream effector of PAX6, in some, but not all lens fibres. FOXE3-positive lens fibres continue to proliferate within the posterior compartment of the embryonic lens, but unlike in the mouse, no proliferation was detected anywhere in the postnatal mole lens. The undifferentiated status of the anterior epithelial cells was compromised, and most of them undergo apoptosis. Furthermore, β-crystallin and PROX1 expression patterns are abnormal and our data suggest that genes encoding β-crystallins are not directly regulated by PAX6, c-MAF and PROX1 in the Iberian mole, as they are in other model vertebrates.

Conclusion

In other model vertebrates, genetic pathways controlling lens development robustly compartmentalise the lens into a simple, undifferentiated, proliferative anterior epithelium, and quiescent, anuclear, terminally differentiated posterior lens fibres. These pathways are not as robust in the mole, and lead to loss of the anterior epithelial phenotype and only partial differentiation of the lens fibres, which continue to express 'epithelial' genes. Paradigms of genetic regulatory networks developed in other vertebrates appear not to hold true for the Iberian mole.

Cubozoan crystallins: evidence for convergent evolution of pax regulatory sequences

Cnidaria is the earliest-branching metazoan phylum containing a well-developed, lens-containing visual system located on specialized sensory structures called rhopalia. Each rhopalium in a cubozoan jellyfish Tripedalia cystophora has a large and a small complex, camera-type eye with a cellular lens containing distinct families of crystallins. Here, we have characterized J2-crystallin and its gene in T. cystophora. The J2-crystallin gene is composed of a single exon and encodes a 157-amino acid cytoplasmic protein with no apparent homology to known proteins from other species. The non-lens expression of J2-crystallin suggests nonoptical as well as crystallin functions consistent with the gene-sharing strategy that has been used during evolution of lens crystallins in other invertebrates and vertebrates. Although nonfunctional in transfected mammalian lens cells, the J2-crystallin promoter is activated by the jellyfish paired domain transcription factor PaxB in co-transfection tests via binding to three paired domain sites. PaxB paired domain-binding sites were also identified in the PaxB-regulated promoters of the J1A- and J1B-crystallin genes, which are not homologous to the J2-crystallin gene. Taken together with previous studies on the regulation of the diverse crystallin genes, the present report strongly supports the idea that crystallin recruitment of multifunctional proteins was driven by convergent changes involving Pax (as well as other transcription factors) in the promoters of nonhomologous genes within and between species as well as within gene families.

Eye evolution: lens and cornea as an upgrade of animal visual system

Lens-containing eyes are a feature of surprisingly broad spectrum of organisms across the animal kingdom that represent a significant improvement of simple eye composed of just photoreceptor cells and pigment cells. It is apparent that such an upgrade of animal visual system has originated numerous times during evolution since many distinct strategies to enhance light refraction through the use of lens and cornea have been utilized. In addition to having an ancient role in prototypical eye formation Pax transcription factors were convergently recruited for regulation of structurally diverse crystallins and genes affecting morphogenesis of various lens-containing eyes.

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.

Protein Phosphatase-1 Modulates the Function of Pax-6, a Transcription Factor Controlling Brain and Eye Development

Pax-6 is an evolutionarily conserved transcription factor and acts high up in the regulatory hierarchy controlling eye and brain development in humans, mice, zebrafish, and Drosophila. Previous studies have shown that Pax-6 is a phosphoprotein, and its phosphorylation by ERK, p38, and homeodomain-interacting protein kinase 2 greatly enhances its transactivation activity. However, the protein phosphatases responsible for the dephosphorylation of Pax-6 remain unknown. Here, we present both in vitro and in vivo evidence to show that protein serine/threonine phosphatase-1 is a major phosphatase that directly dephosphorylates Pax-6. First, purified protein phosphatase-1 directly dephosphorylates Pax-6 in vitro. Second, immunoprecipitation-linked Western blot revealed that both protein phosphatase-1alpha and protein phosphatase-1beta interact with Pax-6. Third, overexpression of protein phosphatase-1 in human lens epithelial cells leads to dephosphorylation of Pax-6. Finally, inhibition of protein phosphatase-1 activity by calyculin A or knockdown of protein phosphatase-1alpha and protein phosphatase-1beta by RNA interference leads to enhanced phosphorylation of Pax-6. Moreover, our results also demonstrate that dephosphorylation of Pax-6 by protein phosphatase-1 significantly modulates its function in regulating expression of both exogenous and endogenous genes. These results demonstrate that protein phosphatase 1 acts as a major phosphatase to dephosphorylate Pax-6 and modulate its function.

Crystallization and preliminary X-ray analysis of the Pax6 paired domain bound to the Pax6 gene enhancer

Pax6 is a member of the Pax family of transcription factors and is essential for eye development. Pax6 has two DNA-binding domains: the paired domain and the homeodomain. The Pax6 paired domain is involved in Pax6 gene autoregulation by binding to its enhancer. In this study, crystallization and preliminary X-ray diffraction analysis of the mammalian Pax6 paired domain in complex with the Pax6 gene enhancer was attempted. The Pax6 paired domain complexed with an optimized 25 bp DNA fragment was crystallized by the hanging-drop vapour-diffusion method. The crystal diffracted synchrotron radiation to 3.0/3.7 A resolution and belongs to the monoclinic space group P2(1), with unit-cell parameters a = 62.21, b = 70.69, c = 176.03 A, beta = 90.54 degrees. Diffraction data were collected to 3.7 A resolution.

Regulation of gene expression by Pax6 in ocular cells: a case of tissue-preferred expression of crystallins in lens

Lens development is an excellent model for genetic and biochemical studies of embryonic induction, cell cycle regulation, cellular differentiation and signal transduction. Differentiation of lens is characterized by lens-preferred expression and accumulation of water-soluble proteins, crystallins. Crystallins are required for light transparency, refraction and maintenance of lens integrity. Here, we review mechanisms of lens-preferred expression of crystallin genes by employing synergism between developmentally regulated DNA-binding transcription factors: Pax6, c-Maf, MafA/L-Maf, MafB, NRL, Sox2, Sox1, RARbeta/RXRbeta, RORalpha, Prox1, Six3, gammaFBP-B and HSF2. These factors are differentially expressed in lens precursor cells, lens epithelium and primary and secondary lens fibers. They exert their function in combination with ubiquitously expressed factors (e.g. AP-1, CREB, pRb, TFIID and USF) and co-activators/chromatin remodeling proteins (e.g. ASC-2 and CBP/p300). A special function belongs to Pax6, a paired domain and homeodomain-containing protein, which is essential for lens formation. Pax6 is expressed in lens progenitor cells before the onset of crystallin expression and it serves as an important regulatory factor required for expression of c-Maf, MafA/L-Maf, Six3, Prox1 and retinoic acid signaling both in lens precursor cells and the developing lens. The roles of these factors are illustrated by promoter studies of mouse alphaA-, alphaB-, gammaF- and guinea pig zeta-crystallins. Pax6 forms functional complexes with a number of transcription factors including the retinoblastoma protein, pRb, MafA, Mitf and Sox2. We present novel data showing that pRb antagonizes Pax6-mediated activation of the alphaA-crystallin promoter likely by inhibiting binding of Pax6 to DNA.

Mafs, Prox1, and Pax6 can regulate chicken betaB1-crystallin gene expression

During lens fiber cell differentiation, the regulation of crystallin gene expression is coupled with dramatic morphological changes. Here we report that Mafs, Prox1, and Pax6, which are essential transcription factors for normal lens development, bind to three functionally important cis elements, PL1, PL2, and OL2, in the chicken betaB1-crystallin promoter and may cooperatively direct the transcription of this lens fiber cell preferred gene. Gel shift assays demonstrated that Mafs bind to the MARE-like sequences in the PL1 and PL2 elements, whereas Prox1, a sequence-specific DNA-binding protein like its Drosophila homolog Prospero, interacts with the OL2 element. Furthermore, Pax6, a known repressor of the chicken betaB1-crystallin promoter, binds to all three of these cis elements. In transfection assays, Mafs and Prox1 activated the chicken betaB1-crystallin promoter; however, their transactivation ability was repressed when co-transfected with Pax6. Taken together with the known spatiotemporal expression patterns of Mafs, Prox1, and Pax6 in the developing lens, we propose that Pax6 occupies and represses the chicken betaB1-crystallin promoter in lens epithelial cells, and is displaced by Prox1 and Mafs, which activate the promoter, in differentiating cortical fiber cells.

Requirement for Pax6 in corneal morphogenesis: a role in adhesion

The Pax6 transcription factor functions early during embryogenesis to control key steps in brain, pancreas, olfactory and ocular system development. A requirement for Pax6 in proper formation of lens, iris and retina is well documented. By examining the corneas of heterozygous Small eye (SEY) mice, this report shows that Pax6 is also necessary for normal corneal morphogenesis. In particular, the epithelial component of the postnatal and adult SEY (+/-) cornea is thinner owing to a reduction in the number of cell layers, despite a tenfold increase in the proliferative index and no change in TUNEL labeling. Ultrastructural views revealed large gaps between corneal epithelial cells and a change in the appearance of desmosomes, suggesting that adhesion abnormalities contribute to the corneal phenotype of SEY (+/-) mice. Western blot analysis and immunofluorescence showed equivalent amounts and normal localization of E-cadherin in SEY (+/-) corneas, and the actin cytoskeleton appeared normal as judged by phalloidin staining. By contrast, the levels of desmoglein, beta-catenin and gamma-catenin were reduced in the SEY (+/-) cornea. In addition, the amount of keratin-12 mRNA and protein, the major intermediate filament, was reduced in SEY (+/-) corneal epithelium as shown by in situ hybridization and immunohistochemistry. Finally, the SEY (+/-) corneal epithelium adheres less well than wild-type when challenged with gentle rubbing using a microsponge. In conclusion, our results indicate that cellular adhesion is compromised in the SEY (+/-) corneal epithelium and suggests a role for Pax6 in the proper generation and maintenance of the adult cornea.

JNK initiates a cytokine cascade that causes Pax2 expression and closure of the optic fissure

The c-Jun NH(2)-terminal kinase (JNK) group of mitogen-activated protein kinases is stimulated in response to a wide array of cellular stresses and proinflammatory cytokines. Mice lacking individual members of the Jnk family (Jnk1, Jnk2, and Jnk3) are viable and survive without overt structural abnormalities. Here we show that mice with a compound deficiency in Jnk expression can survive to birth, but fail to close the optic fissure (retinal coloboma). We demonstrate that JNK initiates a cytokine cascade of bone morphogenetic protein-4 (BMP4) and sonic hedgehog (Shh) that induces the expression of the paired-like homeobox transcription factor Pax2 and closure of the optic fissure. Interestingly, the role of JNK to regulate BMP4 expression during optic fissure closure is conserved in Drosophila during dorsal closure, a related morphogenetic process that requires JNK-regulated expression of the BMP4 ortholog Decapentaplegic (Dpp).

Pax6 autoregulation mediated by direct interaction of Pax6 protein with the head surface ectoderm-specific enhancer of the mouse Pax6 gene

The Pax6 gene plays crucial roles in eye development and encodes a transcription factor containing both a paired domain and a homeodomain. During embryogenesis, Pax6 is expressed in restricted tissues under the direction of distinct cis-regulatory regions. The head surface ectoderm-specific enhancer of mouse Pax6 directs reporter expression in the derivatives of the ectoderm in the eye, such as lens and cornea, but the molecular mechanism of its control remains largely unknown. We identified a Pax6 protein-responsive element termed LE9 (52 bp in length) within the head surface ectoderm-specific enhancer. LE9, a sequence well conserved across vertebrates, acted as a highly effective enhancer in reporter analyses. Pax6 protein formed in vitro a complex with the distal half of LE9 in a manner dependent on the paired domain. The proximal half of the LE9 sequence contains three plausible sites of HMG domain recognition, and HMG domain-containing transcription factors Sox2 and Sox3 activated LE9 synergistically with Pax6. A scanning mutagenesis experiment indicated that the central site is most important among the three presumptive HMG domain recognition sites. Furthermore, Pax6 and Sox2 proteins formed a complex when they were expressed together. Based on these findings, we propose a model in which Pax6 protein directly and positively regulates its own gene expression, and Sox2 and Sox3 proteins interact with Pax6 protein, resulting in modification of the transcriptional activation by Pax6 protein.

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

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

Pax6; a pleiotropic player in development

Pax6 is a transcription factor essential for the development of tissues including the eyes, central nervous system and endocrine glands of vertebrates and invertebrates. It regulates the expression of a broad range of molecules, including transcription factors, cell adhesion and short-range cell-cell signalling molecules, hormones and structural proteins. It has been implicated in a number of key biological processes including cell proliferation, migration, adhesion and signalling both in normal development and in oncogenesis. The mechanisms by which Pax6 regulates its downstream targets likely involve the use of different splice variants and interactions with multiple proteins, allowing it to generate different effects in different cells. Extrapolation to developmental transcription factors in general suggests that variation in the nature of individual factors is likely to contribute to the emergence of differences between tissues.

Neural stem cell heterogeneity demonstrated by molecular phenotyping of clonal neurospheres

Neural stem cells (NSCs) in vitro are able to generate clonal structures, "neurospheres," that exhibit intra-clonal neural cell-lineage diversity; i.e., they contain, in addition to NSCs, neuronal and glial progenitors in different states of differentiation. The present study focuses on a subset of neurospheres derived from fresh clinical specimens of human brain by using an in vitro system that relies on particular growth factors, serum, and anchorage withdrawal. Thirty individual and exemplary cDNA libraries from these neurosphere clones were clustered and rearranged within a panel after characterization of differentially expressed transcripts. The molecular phenotypes that were obtained indicate that clonogenic NSCs in our in vitro system are heterogeneous, with subsets reflecting distinct neural developmental commitments. This approach is useful for the sorting and expansion of NSCs and facilitates the discovery of genes involved in cell proliferation, communication, fate control, and differentiation.

Distinct roles of SOX2, Pax6 and Maf transcription factors in the regulation of lens-specific delta1-crystallin enhancer

BACKGROUND

The eye lens provides a good model for the study of regulation of cell differentiation, in which lens-specific delta1-crystallin expression serves as an indicator of the differentiated state of the cells. It has been indicated that the SOX2, Pax6 and Maf proteins are the major regulators of lens cell differentiation. To clarify the individual roles of these transcription factors, we analysed their participation in regulation of the delta1-crystallin enhancer.

RESULTS

We defined the major binding sites of SOX2, Pax6 and Maf transcription factors in the delta1-crystallin enhancer and assessed the effect of mutations at these sites in the cultured lens epithelial cells and in developing lenses of transgenic mouse embryos. SOX2 (or SOX1/SOX3) is essential for activation of the enhancer under all conditions. Pax6 bound at the deltaEF3 site is required for activation of the enhancer, while Pax6 at the Pax6U site appears to be involved in the Pax6-dependent suppression of the enhancer. In contrast, Maf proteins are only required for high enhancer activity in lens fibre cells.

CONCLUSION

The distinct roles of these transcription factors in the regulation of delta1-crystallin enhancer would tend to indicate their individual functions in lens differentiation. The activity of SOX2 and the related SOX1/3 is essential at all stages of lens development as transcriptional activators. Pax6, although it is required in all steps, probably exerts complex regulatory effects, since it possesses both the potential to activate and repress. The major function of Maf proteins presumably resides in the activation of the genes in lens fibre cells.

Early eye development in vertebrates

This review provides a synthesis that combines data from classical experimentation and recent advances in our understanding of early eye development. Emphasis is placed on the events that underlie and direct neural retina formation and lens induction. Understanding these events represents a longstanding problem in developmental biology. Early interest can be attributed to the curiosity generated by the relatively frequent occurrence of disorders such as cyclopia and anophthalmia, in which dramatic changes in eye development are readily observed. However, it was the advent of experimental embryology at the turn of the century that transformed curiosity into active investigation. Pioneered by investigators such as Spemann and Adelmann, these embryological manipulations have left a profound legacy. Questions about early eye development first addressed using tissue manipulations remain topical as we try to understand the molecular basis of this process.

Ectopically localized HNK-1 epitope perturbs migration of the midbrain neural crest cells in Pax6 mutant rat

Small eye rats, which have a mutation in a gene encoding transcription factor Pax6, exhibit impaired migration of the midbrain neural crest cells, thereby showing severe craniofacial defects. Orthotopic grafting of the midbrain neural crest cells taken from the wild-type into Pax6 mutant embryos has suggested environmental defects along the migratory pathway of the midbrain crest cells. In the present study we found that the HNK-1 carbohydrate epitope was ectopically localized in the frontonasal epithelium of Pax6 mutant embryos. The GlcAT-P gene, encoding an enzyme for the synthesis of the HNK-1 epitope, was also expressed ectopically in the frontonasal epithelium of the mutant. In explant cultures, the migration rate of neural crest cells from the midbrain, but not from the forebrain, was significantly less in HNK-1-coated dishes than in non-coated dishes. These results suggest that the arrested migration of the midbrain crest cells in Pax6 mutant embryos may, at least in part, be due to the inhibitory effect of the HNK-1 epitope ectopically localized in the frontonasal epithelium.

Developmental and functional evidence of a role for Zfhep in neural cell development

The rat Zfhep gene encodes a member of the Zfh family of transcription factors having a homeodomain-like sequence and multiple zinc fingers. We examined expression of Zfhep in the rat forebrain during embryonic and postnatal development. Zfhep mRNA was strongly expressed in the progenitor cells of the ventricular zone around the lateral ventricles on E14 and E16, but showed little expression in cells that had migrated to form the developing cortex. Dual labeling with PCNA demonstrated expression of Zfhep mRNA in proliferating cells. Expression of Zfhep in the ventricular zone decreases during late development as the population of progenitor cells decreases. This pattern is distinctly different from other members of the Zfh family. We also examined the expression of Zfhep protein during retinoic acid-induced neurogenesis of P19 embryonal carcinoma cells. Zfhep is highly expressed in P19 neuroblasts, and expression decreases by the time of morphological neurogenesis. Hence, both P19 cells and embryonic brain demonstrate a loss of Zfhep expression during the transition from proliferating precursor to differentiated neural cells. We investigated a possible link between Zfhep and proliferation by treating human glial cell lines with Zfhep antisense phosphorothioate oligodeoxynucleotides. Two Zfhep antisense oligonucleotides repressed proliferation of either U-138 or U-343 glioblastoma cells more than control oligonucleotides. Based on the expression patterns of Zfhep in vivo and in the P19 cell model of neurogenesis, we suggest that Zfhep may play a role in proliferation or differentiation of neural cells.

The upstream ectoderm enhancer in Pax6 has an important role in lens induction

The Pax6 gene has a central role in development of the eye. We show, through targeted deletion in the mouse, that an ectoderm enhancer in the Pax6 gene is required for normal lens formation. Ectoderm enhancer-deficient embryos exhibit distinctive defects at every stage of lens development. These include a thinner lens placode, reduced placodal cell proliferation, and a small lens pit and lens vesicle. In addition, the lens vesicle fails to separate from the surface ectoderm and the maturing lens is smaller and shows a delay in fiber cell differentiation. Interestingly, deletion of the ectoderm enhancer does not eliminate Pax6 production in the lens placode but results in a diminished level that, in central sections, is apparent primarily on the nasal side. This argues that Pax6 expression in the lens placode is controlled by the ectoderm enhancer and at least one other transcriptional control element. It also suggests that Pax6 enhancers active in the lens placode drive expression in distinct subdomains, an assertion that is supported by the expression pattern of a lacZ reporter transgene driven by the ectoderm enhancer. Interestingly, deletion of the ectoderm enhancer causes loss of expression of Foxe3, a transcription factor gene mutated in the dysgenetic lens mouse. When combined, these data and previously published work allow us to assemble a more complete genetic pathway describing lens induction. This pathway features (1) a pre-placodal phase of Pax6 expression that is required for the activity of multiple, downstream Pax6 enhancers; (2) a later, placodal phase of Pax6 expression regulated by multiple enhancers; and (3) the Foxe3 gene in a downstream position. This pathway forms a basis for future analysis of lens induction mechanism.

Pax6 and SOX2 form a co-DNA-binding partner complex that regulates initiation of lens development

Pax6 is a key transcription factor in eye development, particularly in lens development, but its molecular action has not been clarified. We demonstrate that Pax6 initiates lens development by forming a molecular complex with SOX2 on the lens-specific enhancer elements, e.g., the delta-crystallin minimal enhancer DC5. DC5 shows a limited similarity to the binding consensus sequence of Pax6 and is bound poorly by Pax6 alone. However, Pax6 binds cooperatively with SOX2 to the DC5 sequence, resulting in formation of a high-mobility form of ternary complex in vitro, which correlates with the enhancer activation in vivo. We observed Pax6 and SOX2-interdependent factor occupancy of DC5 in a chromatin environment in vivo, providing the molecular basis of synergistic activation by Pax6 and SOX2. Subtle alterations of the Pax6-binding-site sequence of DC5 or of the inter-binding-sites distance diminished the cooperative binding and caused formation of a non-functional low-mobility form complex, suggesting DNA sequence-guided and protein interaction-induced conformation change of the Pax6 protein. When ectopically expressed in embryo ectoderm, Pax6 and SOX2 in combination activate delta-crystallin gene and elicit lens placode development, indicating that the complex of Pax6 and SOX2 formed on specific DNA sequences is the genetic switch for initiation of lens differentiation.

Antagonistic action of Six3 and Prox1 at the gamma-crystallin promoter

Gamma-crystallin genes are specifically expressed in the eye lens. Their promoters constitute excellent models to analyse tissue-specific gene expression. We investigated murine CRYGE/f promoters of different length in lens epithelial cell lines. The most active fragment extends from position -219 to +37. Computer analysis predicts homeodomain and paired-domain binding sites for all rodent CRYGD/e/f core promoters. As examples, we analysed the effects of Prox1 and Six3, which are considered important transcription factors involved in lens development. Because of endogenous Prox1 expression in N/N1003A cells, a weak stimulation of CRYGE/f promoter activity was found for PROX1. In contrast, PROX1 stimulated the CRYGF promoter 10-fold in CD5A cells without endogenous PROX1. In both cell lines Six3 repressed the CRYGF promoter to 10% of its basal activity. Our cell transfection experiments indicated that CRYG expression increases as Six3 expression decreases. Prox1 and Six3 act antagonistically on regulation of the CRYGD/e/f promoters. Functional assays using randomly mutated gammaF-crystallin promoter fragments define a Six3-responsive element between -101 and -123 and a Prox1-responsive element between -151 and -174. Since Prox1 and Six3 are present at the beginning of lens development, expression of CRYGD/e/f is predicted to remain low at this time. It increases as Six3 expression decreases during ongoing lens development.

A set of Hox proteins interact with the Maf oncoprotein to inhibit its DNA binding, transactivation, and transforming activities

Maf oncoprotein is a basic-leucine zipper (bZip) type of transcriptional activator. Since many transcription factors are known to form functional complexes, we searched for proteins that interact with the DNA-binding domain of Maf using the phage display method and identified two homeodomain-containing proteins, Hoxd12 and MHox/Prx1/Phox1/Pmx1. Studies with mutants of Hox and Maf proteins showed that they associate through their DNA-binding domains; the homeodomain of Hox and the bZip domain of Maf, respectively. Reflecting the high similarity of the bZip domain, all other Maf family members tested (c-/v-Maf, MafB, MafK, MafF, and MafG) also associated with the Hox proteins. Pax6, whose homeodomain is relatively similar to MHox, also could interact with Maf. However, two other bZip oncoproteins, Fos and Jun, failed to associate with the Hox proteins, while a distantly related Hox family member, Meis1, could not interact with Maf. Through interactions with the bZip domain, the Hox proteins inhibited the DNA binding activity of Maf, whereas the binding of Hox proteins to their recognition sequences was not abrogated by Maf. We further showed that coexpression of the Hox proteins repressed transcriptional activation and transforming activity of Maf. These results suggested that the interaction of a set of Hox proteins with Maf family members may interfere not only with their oncogenicity but also with their physiological roles.

Overexpression of PAX6(5a) in lens fiber cells results in cataract and upregulation of (alpha)5(beta)1 integrin expression

The PAX6 gene, a key regulator of eye development, produces two major proteins that differ in paired domain structure: PAX6 and PAX6(5a). It is known that an increase in the PAX6(5a) to PAX6 ratio leads to multiple ocular defects in humans. Here, transgenic mice were created that overexpress human PAX6(5a) in the lens. These mice develop cataracts with abnormalities in fiber cell shape as well as fiber cell/lens capsule and fiber cell/fiber cell interactions. While the structure of the actin cytoskeleton appeared relatively normal, the cataractous lens expresses increased amounts of paxillin and p120(ctn) as well as large aggregates of (alpha)5(beta)1 integrin in the dysgenic fiber cells. The elevated amounts of these proteins in the transgenic lens correlated well with elevated levels of their respective mRNAs. To investigate the role of Pax-6(5a) in the upregulation of these genes, a series of gel shift experiments using truncated proteins and consensus oligonucleotides demonstrated the complexity of Pax-6 and Pax-6(5a) binding to DNA, aiding our identification of potential binding sites in the human (α)5- and (beta)1-integrin promoters. Consequent gel shift analysis demonstrated that these putative regulatory sequences bind Pax-6 and/or Pax-6(5a) in lens nuclear extracts, suggesting that the human (alpha)5 and (beta)1 integrin promoters contain PAX6/PAX6(5a) binding sites and maybe directly regulated by this transcription factor in the transgenic lens. We conclude that these transgenic mice are good models to study a type of human cataract and for identifying batteries of genes that are directly or indirectly regulated by both forms of Pax-6.

Lens-preferred activity of chicken delta 1- and delta 2-crystallin enhancers in transgenic mice and evidence for retinoic acid-responsive regulation of the delta 1-crystallin gene

There are two tandemly linked delta-crystallin genes [5' delta 1 -delta 2 3'] in the chicken, with the delta 1-crystallin gene being expressed much more highly (50-100-fold) in the embryonic lens than the delta 2-crystallin gene. Previous transfection experiments have shown that a lens-preferred enhancer exists in the third intron of each chicken delta-crystallin gene. In the present investigation we have used transgenic mice to establish that both the chicken delta 1- and delta 2-crystallin enhancers are preferentially active in the mouse lens in combination with their homologous promoter and the chloramphenicol acetyltransferase (CAT) reporter gene. The promoter/ CAT constructs lacking the enhancers were inactive in the transgenic mice. In one case, a truncated delta 2-crystallin promoter (-308/+24) in combination with the enhancer was also active in the Purkinje cells of the cerebellum of the transgenic mice, which could prove useful in future experiments. Finally, retinoic acid receptors (RAR beta) activated the delta 1-crystallin, but not the delta 2-crystallin enhancer in teh recombinant plasmids in cotransfected embryonic chicken lens epithelial cells treated with retinoic acid. This activation did not occur when using the care enhancer (fragment B4) lacking surrounding flanking sequences (fragment B3 and B5) of the enhancer. Together these experiments show that the chicken delta-crystallin enhancers show lens-preference in transgenic mice despite the absence of delta-crystallin in this species and add retinoic acid nuclear receptors to the growing list of transcription factors (including Pax-6, Sox-2, and delta EF3) that directly or indirectly contribute to the high expression of the delta 1-crystallin gene in the lens.

Pax-6 expression and activity are induced in the reepithelializing cornea and control activity of the transcriptional promoter for matrix metalloproteinase gelatinase B

Recent evidence supports the idea that matrix metalloproteinases (MMPs) act as morphogenetic regulators in embryonic and adult events of tissue remodeling. MMP activity is controlled primarily at the level of gene expression. In a recent study we characterized the transcriptional promoter of the MMP gene, gelatinase B (gelB), in transgenic mice, demonstrating the requirement for DNA sequences between -522 and +19 for appropriate activity. In this study we investigated factors required for gelB promoter activity in the developing eye and reepithelializing adult cornea. Pax-6 is a homeobox and paired domain transcription factor that acts at the top of the hierarchy of genes controlling eye development. Pax-6 is also expressed in the adult eye. We show here that the tissue expression pattern of Pax-6 overlaps extensively with gelB promoter activity in the developing and adult eye. In addition Pax-6 is observed to be upregulated in repairing corneal epithelium, as is gelB promoter activity. In cell culture transfection experiments, we identified two promoter regions which mediate positive response to Pax-6. By electrophoretic mobility shift assay, we further pinpoint two Pax-6 binding sites within these response regions and demonstrate direct interaction of the Pax-6 paired domain with one of these sites. These data suggest a mechanism by which Pax-6 may direct gelB expression in an eye-specific manner.

Up-regulation of crystallin mRNAs in form-deprived chick eyes

Form-deprivation of chicks during early postnatal development results in ocular enlargement and great myopic refractive error (form-deprivation myopia). Previous studies have indicated that the retina, RPE and choroid play important roles in ocular enlargement in form-deprivation myopia. We aimed to isolate genes up-regulated in the retina-RPE-choroid of form-deprived chick eyes. A suppression subtractive hybridization method was used to compare mRNA expression in the retina-RPE-choroid of form-deprived and control eyes. One up-regulated cDNA was isolated and identified as part of chick delta1-crystallin cDNA. Northern blot and RT-PCR analyses demonstrated that delta1-crystallin mRNA was up-regulated in the retina-RPE at day 7 after form-deprivation treatment. Semi-quantitative RT-PCE analysis of the expression of several transcription factors indicated that Sox1 and Sox3 were upregulated in parallel with delta1-crystallin mRNA in form-deprived eyes. Northern blot analysis demonstrated that alphaA-, betaA3/A1-, betaB1-, and betaB2-crystallin mRNAs were also up-regulated in form-deprived eyes. Although the detailed mechanisms and functions of the crystallin family genes in the retina-RPE-choroid of form-deprived eyes remain unclear, results of our study suggests that form-deprivation affects the expression of these genes in chick eyes.

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

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

Pax-6 and Prox 1 expression during lens regeneration from Cynops iris and Xenopus cornea: evidence for a genetic program common to embryonic lens development

Lens regeneration from non-lens ocular tissues has been well documented in amphibians, from the dorsal iris in the newt and from the outer cornea in Xenopus. To understand the early molecular events which govern lens regeneration, we examined the expression of two early marker genes of normal lens development, Pax-6 and Prox 1. In both Cynops (newt) iris and Xenopus cornea, Pax-6 is expressed soon after lentectomy in a region broader than that giving rise to the regenerating lens, indicative of an important role for Pax-6 in determination of the regeneration potential. Then Prox 1 expression begins within the Pax-6-expressing tissue, and these Prox 1-expressing cells give rise to the regenerating lens. This sequence of events also takes place in the lens placode of the embryo, indicating that the presence of the same genetic program operates in both embryonic lens development and lens regeneration, at least partly. In the Cynops iris, Pax-6 expression occurs initially in the entire marginal region of the iris after lentectomy but then becomes restricted to the dorsal region. Further studies are expected to elucidate the mechanism of this long-standing problem of the dorsal-restriction of lens regeneration from the newt iris.

Direct regulatory interaction of the eyeless protein with an eye-specific enhancer in the sine oculis gene during eye induction in Drosophila

The Pax-6 gene encodes a transcription factor with two DNA-binding domains, a paired and a homeodomain, and is expressed during eye morphogenesis and development of the nervous system. Pax-6 homologs have been isolated from a wide variety of organisms ranging from flatworms to humans. Since loss-of-function mutants in insects and mammals lead to an eyeless phenotype and Pax-6 orthologs from distantly related species are capable of inducing ectopic eyes in Drosophila, we have proposed that Pax-6 is a universal master control gene for eye morphogenesis. To determine the extent of evolutionary conservation of the eye morphogenetic pathway, we have begun to identify subordinate target genes of Pax-6. Previously we have shown that expression of two genes, sine oculis (so) and eyes absent (eya), is induced by eyeless (ey), the Pax-6 homolog of Drosophila. Here we present evidence from ectopic expression studies in transgenic flies, from transcription activation studies in yeast, and from gel shift assays in vitro that the EY protein activates transcription of sine oculis by direct interaction with an eye-specific enhancer in the long intron of the so gene.

Lens regeneration in Xenopus is not a mere repeat of lens development, with respect to crystallin gene expression

The spatio-temporal expression of three crystallin genes (alpha A, beta B1 and gamma) in lenses of Xenopus laevis was studied by in situ hybridization to compare the process of lens formation in embryonic development with that of lens regeneration from cornea that occurs in the tadpole. During embryonic lens development, all three crystallin transcripts were initially detected at the same stage of lens placode formation, and subsequently their signals became restricted to the presumptive lens fiber region. At later stages, the three crystallin genes were expressed in primary and secondary lens fibers, but not in lens epithelium. During lens regeneration, alpha A- and beta B1-crystallin signals were first detected in the presumptive lens fiber region of the lens vesicle. The expression of gamma-crystallin, however, appeared later than the other two crystallin genes and was detected only in morphologically discernible lens fibers. In the later stages of lens regeneration, expression of these crystallins was observed only in the lens fiber region, similar to embryonic lens development. These results reveal that lens regeneration from the inner layer of the outer cornea is not simply a repetition of embryonic lens development, when examined at the level of crystallin gene transcription.

A novel alternative spliced variant of the transcription factor AP2alpha is expressed in the murine ocular lens

The AP2alpha gene encodes a transcription factor containing a basic, helix-span-helix DNA-binding/dimerization domain, which is developmentally regulated and retinoic acid inducible. Recent reports about AP2alpha null mice indicate that AP2alpha plays an important role in embryogenesis, especially in craniofacial development and midline fusion. Ocular development is also affected in these null mice. As AP2alpha may be involved in transcriptional regulation in the lens, it was important to examine the expression of the AP2alpha gene in the lens. Four AP2alpha mRNA variants have been previously isolated from whole mouse embryos. Variants 1, 3, and 4 are transcriptional activators that are transcribed from different promoters and variant 2 is a repressor lacking the activation domain encoded by exon 2. Using in situ-PCR, we found that AP2alpha is expressed in the lens epithelia but not in the lens fibers. RT-PCR analysis of lens mRNA with amplimers specific for each variant revealed that AP2alpha variants 1, 2, and 3 are expressed in newborn mouse lenses. However, variant 4 is not expressed in the lens. In this report we characterized a novel isoform, which we named variant 5, expressed in the lens and kidney. Variant 5, which is generated by alternative splicing, may function as a repressor due to the partial deletion of the proline-rich transactivation domain encoded by exon 2. This is the first molecular characterization of AP2alpha gene expression in the lens. Our results indicate that two activator and two repressor AP2alpha isoforms may play a role in regulating gene expression in the lens.

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

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

Molecular regulators involved in vertebrate eye development

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

Involvement of retinoic acid/retinoid receptors in the regulation of murine alphaB-crystallin/small heat shock protein gene expression in the lens

Crystallins are a diverse group of abundant soluble proteins that are responsible for the refractive properties of the transparent eye lens. We showed previously that Pax-6 can activate the alphaB-crystallin/small heat shock protein promoter via the lens-specific regulatory regions LSR1 (-147/-118) and LSR2 (-78/-46). Here we demonstrate that retinoic acid can induce the accumulation of alphaB-crystallin in N/N1003A lens cells and that retinoic acid receptor heterodimers (retinoic acid receptor/retinoid X receptor; RAR/RXR) can transactivate LSR1 and LSR2 in cotransfection experiments. DNase I footprinting experiments demonstrated that purified RAR/RXR heterodimers will occupy sequences resembling retinoic acid response elements within LSR1 and LSR2. Electrophoretic mobility shift assays using antibodies indicated that LSR1 and LSR2 can interact with endogenous RAR/RXR complexes in extracts of cultured lens cells. Pax-6 and RAR/RXR together had an additive effect on the activation of alphaB-promoter in the transfected lens cells. Thus, the alphaB-crystallin gene is activated by Pax-6 and retinoic acid receptors, making these transcription factors examples of proteins that have critical roles in early development as well as in the expression of proteins characterizing terminal differentiation.

A zinc finger homeodomain transcription factor binds specific thyroid hormone response elements

Thyroid hormone receptors can act through response elements (TREs) having a wide variation of sequence. We screened for transcription factors that bind the rat (r) glycoprotein hormone alpha-subunit TRE (alpha-sub), and isolated a cDNA, termed zinc finger homeodomain enhancer-binding protein (Zfhep), which encodes two separate zinc finger domains (ZD1 and ZD2), and a region similar to homeodomains. DNA-binding assays show that ZD1 or ZD2 can bind the alpha-subunit, rat growth hormone, or thyrotropin beta (TSHbeta) gene TREs, but do not bind DR4 or palindromic (pal) TREs. Methylation interference footprinting demonstrates that Zfhep binds the alpha-sub overlapping the TR-binding site. Similarly, the ZD1 protein footprints over TR-binding halfsites of the rat growth hormone (rGH) and TSHbeta TREs. Hence, Zfhep binding is dependent on sequences within and outside the AGGTCA TR-binding halfsite. Interactions of non-receptor transcription factors (such as Zfhep) with certain TREs are important to modify gene-specific regulation by thyroid hormones.

Overlapping Sp1 and AP2 binding sites in a promoter element of the lens-specific MIP gene

The MIP gene, the founder of the MIP family of channel proteins, is specifically expressed in fiber cells of the ocular lens and expression is regulated temporally and spatially during development. We previously found that a DNA fragment containing 253 bp of 5'-flanking sequence and 42 bp of exon 1 of the human MIP gene contains regulatory elements responsible for lens-specific expression of the MIP gene. In this report we have analyzed the function of overlapping Sp1 and AP2 binding sites present in the MIP promoter. Using DNase I footprinting analysis we found that purified Sp1 and AP2 transcription factors interact with several domains of the human MIP promoter sequence -253/+42. Furthermore, addition of purified Sp1 to Drosophila nuclear extracts activates in vitro transcription from the MIP promoter -253/+42. This promoter activity is competed by oligonucleotides containing domains footprinted with Sp1. Using promoter-reporter gene ( CAT ) constructs we found that the sequence -39/-70 contains a cis regulatory element essential for promoter activity in transient assays in lens cells. EMSA analysis showed that lens nuclear extracts contain factors that bind to the MIP 5'-flanking sequence containing overlapping Sp1 and AP2 binding domains at positions -37/-65. Supershift experiments with lens nuclear extracts indicated that Sp3 is also able to interact with this regulatory element, suggesting that Sp1 and Sp3 may be involved in regulation of transcription of the MIP gene in the lens.