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

Integrated single-cell multiomics uncovers foundational regulatory mechanisms of lens development and pathology

Ocular lens development entails epithelial to fiber cell differentiation, defects in which cause congenital cataracts. We report the first single-cell multiomic atlas of lens development, leveraging snRNA-seq, snATAC-seq and CUT&RUN-seq to discover previously unreported mechanisms of cell fate determination and cataract-linked regulatory networks. A comprehensive profile of cis- and trans-regulatory interactions, including for the cataract-linked transcription factor MAF, is established across a temporal trajectory of fiber cell differentiation. Furthermore, we identify an epigenetic paradigm of cellular differentiation, defined by progressive loss of the H3K27 methylation writer Polycomb repressive complex 2 (PRC2). PRC2 localizes to heterochromatin domains across master-regulator transcription factor gene bodies, suggesting it safeguards epithelial cell fate. Moreover, we demonstrate that FGF hyper-stimulation in vivo leads to MAF network activation and the emergence of novel lens cell states. Collectively, these data depict a comprehensive portrait of lens fiber cell differentiation, while defining regulatory effectors of cell identity and cataract formation.

Integrated single-cell multiomics uncovers foundational regulatory mechanisms of lens development and pathology

Ocular lens development entails epithelial to fiber cell differentiation, defects in which cause congenital cataract. We report the first single-cell multiomic atlas of lens development, leveraging snRNA-seq, snATAC-seq, and CUT&RUN-seq to discover novel mechanisms of cell fate determination and cataract-linked regulatory networks. A comprehensive profile of cis- and trans-regulatory interactions, including for the cataract-linked transcription factor MAF, is established across a temporal trajectory of fiber cell differentiation. Further, we divulge a conserved epigenetic paradigm of cellular differentiation, defined by progressive loss of H3K27 methylation writer Polycomb repressive complex 2 (PRC2). PRC2 localizes to heterochromatin domains across master-regulator transcription factor gene bodies, suggesting it safeguards epithelial cell fate. Moreover, we demonstrate that FGF hyper-stimulation in vivo leads to MAF network activation and the emergence of novel lens cell states. Collectively, these data depict a comprehensive portrait of lens fiber cell differentiation, while defining regulatory effectors of cell identity and cataract formation.

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.

Defect of LSS Disrupts Lens Development in Cataractogenesis

Congenital cataract is one of the leading causes of blindness in children worldwide. About one-third of congenital cataracts are caused by genetic defects. LSS, which encodes lanosterol synthase, is a causal gene for congenital cataracts. LSS is critical in preventing abnormal protein aggregation of various cataract-causing mutant crystallins; however, its roles in lens development remain largely unknown. In our study, we generated a mouse model harboring Lss G589S mutation, which is homologous to cataract-causing G588S mutation in human LSS. Lss^G589S/G589S mice exhibited neonatal lethality at postal day 0 (P0), whereas these mice showed severe opacity in eye lens. Also, we found that cataract was formed at E17.5 after we examined the opacity of embryonic lens from E13.5 to E18.5. Moreover, disrupted lens differentiation occurred at E14.5 prior to formation of the opacity of eye lens, shown as delayed differentiation of lens secondary fiber and disordered lens fiber organization. In addition, RNA-seq analysis indicated that cholesterol synthesis signaling pathways were significantly downregulated. Overall, our findings provide clear evidence that a mouse model harboring a homozygous Lss G589S mutation can recapitulate human congenital cataract. Our study points out that LSS functions as a critical determinant of lens development, which will contribute to better understanding LSS defects in cataractogenesis and developing therapies for cataracts.

Single cell transcriptomics of the developing zebrafish lens and identification of putative controllers of lens development

The vertebrate lens is a valuable model system for investigating the gene expression changes that coordinate tissue differentiation due to its inclusion of two spatially separated cell types, the outer epithelial cells and the deeper denucleated fiber cells that they support. Zebrafish are a useful model system for studying lens development given the organ's rapid development in the first several days of life in an accessible, transparent embryo. While we have strong foundational knowledge of the diverse lens crystallin proteins and the basic gene regulatory networks controlling lens development, no study has detailed gene expression in a vertebrate lens at single cell resolution. Here we report an atlas of lens gene expression in zebrafish embryos and larvae at single cell resolution through five days of development, identifying a number of novel putative regulators of lens development. Our data address open questions about the temperospatial expression of α-crystallins during lens development that will support future studies of their function and provide the first detailed view of β- and γ-crystallin expression in and outside the lens. We describe divergent expression in transcription factor genes that occur as paralog pairs in the zebrafish. Finally, we examine the expression dynamics of cytoskeletal, membrane associated, RNA-binding, and transcription factor genes, identifying a number of novel patterns. Overall these data provide a foundation for identifying and characterizing lens developmental regulatory mechanisms and revealing targets for future functional studies with potential therapeutic impact.

Crystallin gene expression: Insights from studies of transcriptional bursting

Cellular differentiation is marked by temporally and spatially regulated gene expression. The ocular lens is one of the most powerful mammalian model system since it is composed from only two cell subtypes, called lens epithelial and fiber cells. Lens epithelial cells differentiate into fiber cells through a series of spatially and temporally orchestrated processes, including massive production of crystallins, cellular elongation and the coordinated degradation of nuclei and other organelles. Studies of transcriptional and posttranscriptional gene regulatory mechanisms in lens provide a wide range of opportunities to understand global molecular mechanisms of gene expression as steady-state levels of crystallin mRNAs reach very high levels comparable to globin genes in erythrocytes. Importantly, dysregulation of crystallin gene expression results in lens structural abnormalities and cataracts. The mRNA life cycle is comprised of multiple stages, including transcription, splicing, nuclear export into cytoplasm, stabilization, localization, translation and ultimate decay. In recent years, development of modern mRNA detection methods with single molecule and single cell resolution enabled transformative studies to visualize the mRNA life cycle to generate novel insights into the sequential regulatory mechanisms of gene expression during embryogenesis. This review is focused on recent major advancements in studies of transcriptional bursting in differentiating lens fiber cells, analysis of nascent mRNA expression from bi-directional promoters, transient nuclear accumulation of specific mRNAs, condensation of chromatin prior lens fiber cell denucleation, and outlines future studies to probe the interactions of individual mRNAs with specific RNA-binding proteins (RBPs) in the cytoplasm and regulation of translation and mRNA decay.

The cataract-linked RNA-binding protein Celf1 post-transcriptionally controls the spatiotemporal expression of the key homeodomain transcription factors Pax6 and Prox1 in lens development

The homeodomain transcription factors (TFs) Pax6 (OMIM: 607108) and Prox1 (OMIM: 601546) critically regulate gene expression in lens development. While PAX6 mutations in humans can cause cataract, aniridia, microphthalmia, and anophthalmia, among other defects, Prox1 deletion in mice causes severe lens abnormalities, in addition to other organ defects. Furthermore, the optimal dosage/spatiotemporal expression of these key TFs is essential for development. In lens development, Pax6 expression is elevated in cells of the anterior epithelium compared to fiber cells, while Prox1 exhibits the opposite pattern. Whether post-transcriptional regulatory mechanisms control these precise TF expression patterns is unknown. Here, we report the unprecedented finding that the cataract-linked RNA-binding protein (RBP), Celf1 (OMIM: 601074), post-transcriptionally regulates Pax6 and Prox1 protein expression in lens development. Immunostaining shows that Celf1 lens-specific conditional knockout (Celf1^cKO) mice exhibit abnormal elevation of Pax6 protein in fiber cells and abnormal Prox1 protein levels in epithelial cells-directly opposite to their normal expression patterns in development. Furthermore, RT-qPCR shows no change in Pax6 and Prox1 transcript levels in Celf1^cKO lenses, suggesting that Celf1 regulates these TFs on the translational level. Indeed, RNA-immunoprecipitation assays using Celf1 antibody indicate that Celf1 protein binds to Pax6 and Prox1 transcripts. Furthermore, reporter assays in Celf1 knockdown and Celf1-overexpression cells demonstrate that Celf1 negatively controls Pax6 and Prox1 translation via their 3' UTRs. These data define a new mechanism of RBP-based post-transcriptional regulation that enables precise control over spatiotemporal expression of Pax6 and Prox1 in lens development, thereby uncovering a new etiological mechanism for Celf1 deficiency-based cataract.

Profiling of chromatin accessibility and identification of general cis-regulatory mechanisms that control two ocular lens differentiation pathways

BACKGROUND

Promoters and enhancers are cis-regulatory DNA sequences that control specificity and quantity of transcription. Both are rich on clusters of cis-acting sites that interact with sequence-specific DNA-binding transcription factors (TFs). At the level of chromatin, these regions display increased nuclease sensitivity, reduced nucleosome density, including nucleosome-free regions, and specific combinations of posttranslational modifications of core histone proteins. Together, "open" and "closed" chromatins represent transcriptionally active and repressed states of individual genes, respectively. Cellular differentiation is marked by changes in local chromatin structure. Lens morphogenesis, regulated by TF Pax6, includes differentiation of epithelial precursor cells into lens fibers in parallel with differentiation of epithelial precursors into the mature lens epithelium.

RESULTS

Using ATAC-seq, we investigated dynamics of chromatin changes during mouse lens fibers and epithelium differentiation. Tissue-specific features of these processes are demonstrated via comparative studies of embryonic stem cells, forebrain, and liver chromatins. Unbiased analysis reveals cis-regulatory logic of lens differentiation through known (e.g., AP-1, Ets, Hsf4, Maf, and Pax6 sites) and novel (e.g., CTCF, Tead, and NF1) motifs. Twenty-six DNA-binding TFs, recognizing these cis-motifs, are markedly up-regulated in differentiating lens fibers. As specific examples, our ATAC-seq data uncovered both the regulatory regions and TF binding motifs in Foxe3, Prox1, and Mip loci that are consistent with previous, though incomplete, experimental data. A cross-examination of Pax6 binding with ATAC-seq data demonstrated that Pax6 bound to both open (H3K27ac and P300-enriched) and closed chromatin domains in lens and forebrain.

CONCLUSIONS

Our study has generated the first lens chromatin accessibility maps that support a general model of stage-specific chromatin changes associated with transcriptional activities of batteries of genes required for lens fiber cell formation. Analysis of active (or open) promoters and enhancers reveals important cis-DNA motifs that establish the molecular foundation for temporally and spatially regulated gene expression in lens. Together, our data and models open new avenues for the field to conduct mechanistic studies of transcriptional control regions, reconstruction of gene regulatory networks that govern lens morphogenesis, and identification of cataract-causing mutations in noncoding sequences.

Eye organogenesis: A hierarchical view of ocular development

This chapter provides an overview of the early developmental origins of six ocular tissues: the cornea, lens, ciliary body, iris, neural retina, and retina pigment epithelium. Many of these tissue types are concurrently specified and undergo a complex set of morphogenetic movements that facilitate their structural interconnection. Within the context of vertebrate eye organogenesis, we also discuss the genetic hierarchies of transcription factors and signaling pathways that regulate growth, patterning, cell type specification and differentiation.

Mutation update of transcription factor genes FOXE3, HSF4, MAF, and PITX3 causing cataracts and other developmental ocular defects

Mutations in the transcription factor genes FOXE3, HSF4, MAF, and PITX3 cause congenital lens defects including cataracts that may be accompanied by defects in other components of the eye or in nonocular tissues. We comprehensively describe here all the variants in FOXE3, HSF4, MAF, and PITX3 genes linked to human developmental defects. A total of 52 variants for FOXE3, 18 variants for HSF4, 20 variants for MAF, and 19 variants for PITX3 identified so far in isolated cases or within families are documented. This effort reveals FOXE3, HSF4, MAF, and PITX3 to have 33, 16, 18, and 7 unique causal mutations, respectively. Loss-of-function mutant animals for these genes have served to model the pathobiology of the associated human defects, and we discuss the currently known molecular function of these genes, particularly with emphasis on their role in ocular development. Finally, we make the detailed FOXE3, HSF4, MAF, and PITX3 variant information available in the Leiden Online Variation Database (LOVD) platform at https://www.LOVD.nl/FOXE3, https://www.LOVD.nl/HSF4, https://www.LOVD.nl/MAF, and https://www.LOVD.nl/PITX3. Thus, this article informs on key variants in transcription factor genes linked to cataract, aphakia, corneal opacity, glaucoma, microcornea, microphthalmia, anterior segment mesenchymal dysgenesis, and Ayme-Gripp syndrome, and facilitates their access through Web-based databases.

Signaling and Gene Regulatory Networks in Mammalian Lens Development

Ocular lens development represents an advantageous system in which to study regulatory mechanisms governing cell fate decisions, extracellular signaling, cell and tissue organization, and the underlying gene regulatory networks. Spatiotemporally regulated domains of BMP, FGF, and other signaling molecules in late gastrula-early neurula stage embryos generate the border region between the neural plate and non-neural ectoderm from which multiple cell types, including lens progenitor cells, emerge and undergo initial tissue formation. Extracellular signaling and DNA-binding transcription factors govern lens and optic cup morphogenesis. Pax6, c-Maf, Hsf4, Prox1, Sox1, and a few additional factors regulate the expression of the lens structural proteins, the crystallins. Extensive crosstalk between a diverse array of signaling pathways controls the complexity and order of lens morphogenetic processes and lens transparency.

Large Maf Transcription Factors: Cousins of AP-1 Proteins and Important Regulators of Cellular Differentiation

A large number of mammalian transcription factors possess the evolutionary conserved basic and leucine zipper domain (bZIP). The basic domain interacts with DNA while the leucine zipper facilitates homo- and hetero-dimerization. These factors can be grouped into at least seven families: AP-1, ATF/CREB, CNC, C/EBP, Maf, PAR, and virus-encoded bZIPs. Here, we focus on a group of four large Maf proteins: MafA, MafB, c-Maf, and NRL. They act as key regulators of terminal differentiation in many tissues such as bone, brain, kidney, lens, pancreas, and retina, as well as in blood. The DNA-binding mechanism of large Mafs involves cooperation between the basic domain and an adjacent ancillary DNA-binding domain. Many genes regulated by Mafs during cellular differentiation use functional interactions between the Pax/Maf, Sox/Maf, and Ets/Maf promoter and enhancer modules. The prime examples are crystallin genes in lens and glucagon and insulin in pancreas. Novel roles for large Mafs emerged from studying generations of MafA and MafB knockouts and analysis of combined phenotypes in double or triple null mice. In addition, studies of this group of factors in invertebrates revealed the evolutionarily conserved function of these genes in the development of multicellular organisms.

Prox1 and fibroblast growth factor receptors form a novel regulatory loop controlling lens fiber differentiation and gene expression

Lens epithelial cells differentiate into lens fibers (LFs) in response to a fibroblast growth factor (FGF) gradient. This cell fate decision requires the transcription factor Prox1, which has been hypothesized to promote cell cycle exit in differentiating LF cells. However, we find that conditional deletion of Prox1 from mouse lenses results in a failure in LF differentiation despite maintenance of normal cell cycle exit. Instead, RNA-seq demonstrated that Prox1 functions as a global regulator of LF cell gene expression. Intriguingly, Prox1 also controls the expression of fibroblast growth factor receptors (FGFRs) and can bind to their promoters, correlating with decreased downstream signaling through MAPK and AKT in Prox1 mutant lenses. Further, culturing rat lens explants in FGF increased their expression of Prox1, and this was attenuated by the addition of inhibitors of MAPK. Together, these results describe a novel feedback loop required for lens differentiation and morphogenesis, whereby Prox1 and FGFR signaling interact to mediate LF differentiation in response to FGF.

Identification of in vivo DNA-binding mechanisms of Pax6 and reconstruction of Pax6-dependent gene regulatory networks during forebrain and lens development

The transcription factor Pax6 is comprised of the paired domain (PD) and homeodomain (HD). In the developing forebrain, Pax6 is expressed in ventricular zone precursor cells and in specific subpopulations of neurons; absence of Pax6 results in disrupted cell proliferation and cell fate specification. Pax6 also regulates the entire lens developmental program. To reconstruct Pax6-dependent gene regulatory networks (GRNs), ChIP-seq studies were performed using forebrain and lens chromatin from mice. A total of 3514 (forebrain) and 3723 (lens) Pax6-containing peaks were identified, with ∼70% of them found in both tissues and thereafter called 'common' peaks. Analysis of Pax6-bound peaks identified motifs that closely resemble Pax6-PD, Pax6-PD/HD and Pax6-HD established binding sequences. Mapping of H3K4me1, H3K4me3, H3K27ac, H3K27me3 and RNA polymerase II revealed distinct types of tissue-specific enhancers bound by Pax6. Pax6 directly regulates cortical neurogenesis through activation (e.g. Dmrta1 and Ngn2) and repression (e.g. Ascl1, Fezf2, and Gsx2) of transcription factors. In lens, Pax6 directly regulates cell cycle exit via components of FGF (Fgfr2, Prox1 and Ccnd1) and Wnt (Dkk3, Wnt7a, Lrp6, Bcl9l, and Ccnd1) signaling pathways. Collectively, these studies provide genome-wide analysis of Pax6-dependent GRNs in lens and forebrain and establish novel roles of Pax6 in organogenesis.

Lens Development and Crystallin Gene Expression

The eye and lens represent excellent models to understand embryonic development at cellular and molecular levels. Initial 3D formation of the eye depends on a reciprocal invagination of the lens placode/optic vesicle to form the eye primordium, i.e., the optic cup partially surrounding the lens vesicle. Subsequently, the anterior part of the lens vesicle gives rise to the lens epithelium, while the posterior cells of the lens vesicle differentiate into highly elongated lens fibers. Lens fiber differentiation involves cytoskeletal rearrangements, cellular elongation, accumulation of crystallin proteins, production of extracellular matrix for the lens capsule, and degradation of organelles. This chapter summarizes recent advances in lens development and provides insights into the regulatory mechanisms and differentiation at the level of chromatin structure and dynamics, the emerging field of noncoding RNAs, and novel strategies to fill the gaps in our understanding of lens development.

The Zeb proteins δEF1 and Sip1 may have distinct functions in lens cells following cataract surgery

PURPOSE

Posterior capsular opacification (PCO), the most prevalent side effect of cataract surgery, occurs when residual lens epithelial cells (LECs) undergo fiber cell differentiation or epithelial-to-mesenchymal transition (EMT). Here, we used a murine cataract surgery model to investigate the role of the Zeb proteins, Smad interacting protein 1 (Sip1) and δ-crystallin enhancer-binding factor 1 (δEF1), during PCO.

METHODS

Extracapsular extraction of lens fiber cells was performed on wild-type and Sip1 knockout mice. Protein expression patterns were assessed at multiple time points after surgery using confocal immunofluorescence. βB1-Crystallin mRNA levels were measured using quantitative RT-PCR. We used Transfac searches to identify δEF1 binding sites in the βB1-crystallin promoter and transfection analysis to test the ability of δEF1 to regulate βB1-crystallin expression.

RESULTS

δEF1, which, in other systems, can activate fibrotic genes (e.g., α-smooth muscle actin) and repress epithelial genes, upregulates by 48 hours after fiber cell removal. In culture, δEF1 repressed βB1-crystallin promoter activity, suggesting that it may also turn off lens gene expression following surgery, contributing to "fibrotic PCO" development. Sip1 also upregulates in LECs by 48 hours, but analysis of Sip1 knockout lenses demonstrated that Sip1 does not play a major role in EMT or fiber cell differentiation after surgery. However, Sip1 knockout LECs do express the ectodermal marker keratin 8, suggesting that Sip1 may limit the reprogramming of residual LECs to an embryonic state.

CONCLUSIONS

Zeb transcription factors likely play important, but distinct roles in PCO development after cataract surgery.

Prox1 ablation in hepatic progenitors causes defective hepatocyte specification and increases biliary cell commitment

The liver has multiple functions that preserve homeostasis. Liver diseases are debilitating, costly and often result in death. Elucidating the developmental mechanisms that establish the liver's architecture or generate the cellular diversity of this organ should help advance the prevention, diagnosis and treatment of hepatic diseases. We previously reported that migration of early hepatic precursors away from the gut epithelium requires the activity of the homeobox gene Prox1. Here, we show that Prox1 is a novel regulator of cell differentiation and morphogenesis during hepatogenesis. Prox1 ablation in bipotent hepatoblasts dramatically reduced the expression of multiple hepatocyte genes and led to very defective hepatocyte morphogenesis. As a result, abnormal epithelial structures expressing hepatocyte and cholangiocyte markers or resembling ectopic bile ducts developed in the Prox1-deficient liver parenchyma. By contrast, excessive commitment of hepatoblasts into cholangiocytes, premature intrahepatic bile duct morphogenesis, and biliary hyperplasia occurred in periportal areas of Prox1-deficient livers. Together, these abnormalities indicate that Prox1 activity is necessary to correctly allocate cell fates in liver precursors. These results increase our understanding of differentiation anomalies in pathological conditions and will contribute to improving stem cell protocols in which differentiation is directed towards hepatocytes and cholangiocytes.

Zebrafish yap1 plays a role in differentiation of hair cells in posterior lateral line

The evolutionarily conserved Hippo signaling pathway controls organ size by regulating cell proliferation and apoptosis and this process involves Yap1. The zebrafish Yap1 acts during neural differentiation, but its function is not fully understood. The detailed analysis of yap1 expression in proliferative regions, revealed it in the otic placode that gives rise to the lateral line system affected by the morpholino-mediated knockdown of Yap1. The comparative microarray analysis of transcriptome of Yap1-deficient embryos demonstrated changes in expression of many genes, including the Wnt signaling pathway and, in particular, prox1a known for its role in development of mechanoreceptors in the lateral line. The knockdown of Yap1 causes a deficiency of differentiation of mechanoreceptors, and this defect can be rescued by prox1a mRNA. Our studies revealed a role of Yap1 in regulation of Wnt signaling pathway and its target Prox1a during differentiation of mechanosensory cells.

Histone posttranslational modifications and cell fate determination: lens induction requires the lysine acetyltransferases CBP and p300

Lens induction is a classical embryologic model to study cell fate determination. It has been proposed earlier that specific changes in core histone modifications accompany the process of cell fate specification and determination. The lysine acetyltransferases CBP and p300 function as principal enzymes that modify core histones to facilitate specific gene expression. Herein, we performed conditional inactivation of both CBP and p300 in the ectodermal cells that give rise to the lens placode. Inactivation of both CBP and p300 resulted in the dramatic discontinuation of all aspects of lens specification and organogenesis, resulting in aphakia. The CBP/p300(-/-) ectodermal cells are viable and not prone to apoptosis. These cells showed reduced expression of Six3 and Sox2, while expression of Pax6 was not upregulated, indicating discontinuation of lens induction. Consequently, expression of αB- and αA-crystallins was not initiated. Mutant ectoderm exhibited markedly reduced levels of histone H3 K18 and K27 acetylation, subtly increased H3 K27me3 and unaltered overall levels of H3 K9ac and H3 K4me3. Our data demonstrate that CBP and p300 are required to establish lens cell-type identity during lens induction, and suggest that posttranslational histone modifications are integral to normal cell fate determination in the mammalian lens.

A crystallin gene network in the mouse retina

The present study was designed to examine the regulation of crystallin genes and protein in the mouse retina using the BXD recombinant inbred (RI) strains. Illumina Sentrix BeadChip Arrays (MouseWG-6v2) were used to analyze mRNA levels in 75 BXD RI strains along with the parental strains (C57Bl/6J and DBA/2J), and the reciprocal crosses in the Hamilton Eye Institute (HEI) Retina Dataset (www.genenetwork.org). Protein levels were investigated using immunoblots to quantify levels of proteins and indirect immunohistochemistry to define the distribution of protein. Algorithms in the Genomatix program were used to identify transcription factor binding sites common to the regulatory sequences in the 5' regions of co-regulated set of crystallin and other genes as compared to a set of control genes. As subset of genes, including many encoding lens crystallins is part of a tightly co-regulated network that is active in the retina. Expression of this crystallin network appears to be binary in nature, being expressed either at relatively low levels or being highly upregulated. Relative to a control set of genes, the 5' regulatory sequences of the crystallin network genes show an increased frequency of a set of common transcription factor-binding sites, the most common being those of the Maf family. Chromatin immunoprecipitation of human lens epithelial cells (HLEC) and rat retinal ganglion cells (RGC) confirmed the functionality of these sites, showing that MafA binds the predicted sites of CRYGA and CRYGD in HLE and CRYAB, CRYGA, CRYBA1, and CRYBB3 in RGC cells. In the retina there is a highly correlated group of genes containing many members of the α- β- and γ-crystallin families. These genes can be dramatically upregulated in the retina. One transcription factor that appears to be involved in this coordinated expression is the MAF family transcription of factors associated with both lens and extralenticular expression of crystallin genes.

Cellular origin of Kaposi's sarcoma and Kaposi's sarcoma-associated herpesvirus-induced cell reprogramming

Kaposi's sarcoma (KS) is the most common malignancy in untreated HIV patients. KS is characterised by abnormal neoangiogenesis, inflammation, and proliferation of tumour cells [KS spindle cells (SCs)]. Kaposi's sarcoma-associated herpesvirus (KSHV) is the aetiological agent of KS. KS SCs are the predominant KSHV-infected cells in KS lesions. In this review, we report advances in understanding of the cellular origin of the KS SC, a contentious topic in KSHV research. KS SCs are now known to be of endothelial cell (EC) origin, phenotypically most similar to lymphatic ECs (LECs), but poorly differentiated. We focus on recent insights into KSHV's ability to exploit the normal differentiation pathway and intrinsic plasticity of ECs, through manipulation of EC-specific transcriptional regulators [i.e., prospero homeobox 1 (PROX1) and MAF] and discuss how this may contribute to viral persistence and KS sarcomagenesis.

Characterization of an apparently novel β-cell line-enriched 80-88 kDa transcriptional activator of the MafA and Pdx1 genes

MafA and Pdx1 represent critical transcriptional regulators required for the maintenance of pancreatic islet β-cell function. The in vivo β-cell-enriched expression pattern of these genes is principally directed by islet transcription factors binding within conserved Region 3 (base pairs (bp) -8118/-7750) of MafA and Area II (bp -2153/-1923) of the Pdx1 gene. Comprehensive mutational analysis of conserved MafA Region 3 revealed two new β-cell line-specific cis-activation elements, termed Site 4 (bp -7997 to -7988) and Site 12 (bp -7835 to -7826). Gel mobility and antibody super-shift analysis identified Pdx1 as the Site 4 binding factor, while an 80-88 kilodalton (kDa) β-cell line-enriched protein complex bound Site 12 and similar aligned nucleotides within Pdx1 Area II. The 80-88 kDa activator was also found in adult mouse islet extract. Strikingly, the molecular weight, DNA binding, and antibody recognition properties of this activator were unique when compared with all other key islet transcription factors tested, including Prox1 (83 kDa), Hnf1α (67 kDa), FoxA2 (48 kDa), MafA (46 kDa), Isl1 (44 kDa), Pdx1 (42 kDa), and Nkx2.2 (30 kDa). Collectively, these data define an apparently novel MafA Region 3 and Pdx1 Area II activator contributing to expression in β-cells.

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.

Microarray-based identification of Pitx3 targets during Xenopus embryogenesis

BACKGROUND

Unexpected phenotypes resulting from morpholino-mediated translational knockdown of Pitx3 in Xenopus laevis required further investigation regarding the genetic networks in which the gene might play a role. Microarray analysis was, therefore, used to assess global transcriptional changes downstream of Pitx3.

RESULTS

From the large data set generated, selected candidate genes were confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization.

CONCLUSIONS

We have identified four genes as likely direct targets of Pitx3 action: Pax6, β Crystallin-b1 (Crybb1), Hes7.1, and Hes4. Four others show equivocal promise worthy of consideration: Vent2, and Ripply2 (aka Ledgerline or Stripy), eFGF and RXRα. We also describe the expression pattern of additional and novel genes that are Pitx3-sensitive but that are unlikely to be direct targets.

Transcriptional regulation of small HSP-HSF1 and beyond

The members of the small heat shock protein (sHSP) family are molecular chaperones that play major roles in development, stress responses, and diseases, and have been envisioned as targets for therapy, particularly in cancer. The molecular mechanisms that regulate their transcription, in normal, stress, or pathological conditions, are characterized by extreme complexity and subtlety. Although historically linked to the heat shock transcription factors (HSFs), the stress-induced or developmental expression of the diverse members, including HSPB1/Hsp27/Hsp25, αA-crystallin/HSPB4, and αB-crystallin/HSPB5, relies on the combinatory effects of many transcription factors. Coupled with remarkably different cis-element architectures in the sHsp regulatory regions, they confer to each member its developmental expression or stress-inducibility. For example, multiple regulatory pathways coordinate the spatio-temporal expression of mouse αA-, αB-crystallin, and Hsp25 genes during lens development, through the action of master genes, like the large Maf family proteins and Pax6, but also HSF4. The inducibility of Hsp27 and αB-crystallin transcription by various stresses is exerted by HSF-dependent mechanisms, by which concomitant induction of Hsp27 and αB-crystallin expression is observed. In contrast, HSF-independent pathways can lead to αB-crystallin expression, but not to Hsp27 induction. Not surprisingly, deregulation of the expression of sHSP is associated with various pathologies, including cancer, neurodegenerative, or cardiac diseases. However, many questions remain to be addressed, and further elucidation of the developmental mechanisms of sHsp gene transcription might help to unravel the tissue- and stage-specific functions of this fascinating class of proteins, which might prove to be crucial for future therapeutic strategies. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Association of high myopia with crystallin beta A4 (CRYBA4) gene polymorphisms in the linkage-identified MYP6 locus

Background

Myopia is the most common ocular disorder worldwide and imposes tremendous burden on the society. It is a complex disease. The MYP6 locus at 22 q12 is of particular interest because many studies have detected linkage signals at this interval. The MYP6 locus is likely to contain susceptibility gene(s) for myopia, but none has yet been identified.

Methodology/Principal Findings

Two independent subject groups of southern Chinese in Hong Kong participated in the study an initial study using a discovery sample set of 342 cases and 342 controls, and a follow-up study using a replication sample set of 316 cases and 313 controls. Cases with high myopia were defined by spherical equivalent ≤ -8 dioptres and emmetropic controls by spherical equivalent within ±1.00 dioptre for both eyes. Manual candidate gene selection from the MYP6 locus was supported by objective in silico prioritization. DNA samples of discovery sample set were genotyped for 178 tagging single nucleotide polymorphisms (SNPs) from 26 genes. For replication, 25 SNPs (tagging or located at predicted transcription factor or microRNA binding sites) from 4 genes were subsequently examined using the replication sample set. Fisher P value was calculated for all SNPs and overall association results were summarized by meta-analysis. Based on initial and replication studies, rs2009066 located in the crystallin beta A4 (CRYBA4) gene was identified to be the most significantly associated with high myopia (initial study: P = 0.02; replication study: P = 1.88e-4; meta-analysis: P = 1.54e-5) among all the SNPs tested. The association result survived correction for multiple comparisons. Under the allelic genetic model for the combined sample set, the odds ratio of the minor allele G was 1.41 (95% confidence intervals, 1.21-1.64).

Conclusions/Significance

A novel susceptibility gene (CRYBA4) was discovered for high myopia. Our study also signified the potential importance of appropriate gene prioritization in candidate selection.

Activation of the hedgehog signaling pathway in the developing lens stimulates ectopic FoxE3 expression and disruption in fiber cell differentiation

PURPOSE

The signaling pathways and transcriptional effectors responsible for directing mammalian lens development provide key regulatory molecules that can inform our understanding of human eye defects. The hedgehog genes encode extracellular signaling proteins responsible for patterning and tissue formation during embryogenesis. Signal transduction of this pathway is mediated through activation of the transmembrane proteins smoothened and patched, stimulating downstream signaling resulting in the activation or repression of hedgehog target genes. Hedgehog signaling is implicated in eye development, and defects in hedgehog signaling components have been shown to result in defects of the retina, iris, and lens.

METHODS

We assessed the consequences of constitutive hedgehog signaling in the developing mouse lens using Cre-LoxP technology to express the conditional M2 smoothened allele in the embryonic head and lens ectoderm.

RESULTS

Although initial lens development appeared normal, morphological defects were apparent by E12.5 and became more significant at later stages of embryogenesis. Altered lens morphology correlated with ectopic expression of FoxE3, which encodes a critical gene required for human and mouse lens development. Later, inappropriate expression of the epithelial marker Pax6, and as well as fiber cell markers c-maf and Prox1 also occurred, indicating a failure of appropriate lens fiber cell differentiation accompanied by altered lens cell proliferation and cell death.

CONCLUSIONS

Our findings demonstrate that the ectopic activation of downstream effectors of the hedgehog signaling pathway in the mouse lens disrupts normal fiber cell differentiation by a mechanism consistent with a sustained epithelial cellular developmental program driven by FoxE3.

Pancreas-specific deletion of Prox1 affects development and disrupts homeostasis of the exocrine pancreas

BACKGROUND & AIMS

The exocrine portion of the pancreas functions in digestion and preserves pancreatic homeostasis. Learning how this tissue forms during embryogenesis could improve our understanding of human pancreatic diseases. Expression of the homeobox gene Prox1 in the exocrine pancreas changes throughout development in mice. We investigated the role of Prox1 in development of the exocrine pancreas in mice.

METHODS

Mice with pancreas-specific deletion of Prox1 (Prox1(ΔPanc)) were generated and their pancreatic tissues were analyzed using immunohistochemistry, transmission electron microscopy, histologic techniques, quantitative real-time polymerase chain reaction, immunoblotting, and morphometric analysis.

RESULTS

Loss of Prox1 from the pancreas led to multiple exocrine alterations, most notably premature acinar cell differentiation, increased ductal cell proliferation, altered duct morphogenesis, and imbalanced expression of claudin proteins. Prox1(ΔPanc) mice also had some minor alterations in islet cells, but beta-cell development was not affected. The exocrine congenital defects of Prox1(ΔPanc) pancreata appeared to initiate a gradual process of deterioration that resulted in extensive loss of acinar cells, lipomatosis, and damage to ductal tissue in adult mice.

CONCLUSIONS

Pancreas-specific deletion of Prox1 causes premature differentiation of acinar cells and poor elongation of epithelial branches; these defects indicate that Prox1 controls the expansion of tip progenitors in the early developing pancreas. During later stages of embryogenesis, Prox1 appears to regulate duct cell proliferation and morphogenesis. These findings identify Prox1 as an important regulator of pancreatic exocrine development.

The lens in focus: a comparison of lens development in Drosophila and vertebrates

The evolution of the eye has been a major subject of study dating back centuries. The advent of molecular genetics offered the surprising finding that morphologically distinct eyes rely on conserved regulatory gene networks for their formation. While many of these advances often stemmed from studies of the compound eye of the fruit fly, Drosophila melanogaster, and later translated to discoveries in vertebrate systems, studies on vertebrate lens development far outnumber those in Drosophila. This may be largely historical, since Spemann and Mangold's paradigm of tissue induction was discovered in the amphibian lens. Recent studies on lens development in Drosophila have begun to define molecular commonalities with the vertebrate lens. Here, we provide an overview of Drosophila lens development, discussing intrinsic and extrinsic factors controlling lens cell specification and differentiation. We then summarize key morphological and molecular events in vertebrate lens development, emphasizing regulatory factors and networks strongly associated with both systems. Finally, we provide a comparative analysis that highlights areas of research that would help further clarify the degree of conservation between the formation of dioptric systems in invertebrates and vertebrates.

Characterization of Prox1 and VEGFR-3 expression and lymphatic phenotype in normal organs of mice lacking p50 subunit of NF-κB

OBJECTIVE

Inflammation and NF-κB are highly associated with lymphangiogenesis but the underlying mechanisms remain unclear. We recently established that activated NF-κB p50 subunit increases expression of the main lymphangiogenic mediators, VEGFR-3 and its transcriptional activator, Prox1. To elucidate the role of p50 in lymphatic vasculature, we compared LVD and phenotype in p50 KO and WT mice.

METHODS

Normal tissues from KO and WT mice were stained for LYVE-1 to calculate LVD. VEGFR-3 and Prox1 expressions were analyzed by immunofluorescence and qRT-PCR.

RESULTS

Compared with WT, LVD in the liver and lungs of KO mice was reduced by 39% and 13%, respectively. This corresponded to 25-44% decreased VEGFR-3 and Prox1 expression. In the MFP, LVD was decreased by 18% but VEGFR-3 and Prox1 expression was 80-140% higher than in WT. Analysis of p65 and p52 NF-κB subunits and an array of inflammatory mediators showed a significant increase in p50 alternative pathways in the MFP but not in other organs.

CONCLUSIONS

These findings demonstrate the role of NF-κB p50 in regulating the expression of VEGFR-3, Prox1 and LVD in the mammary tissue, liver, and lung.

Molecular and cellular aspects of amphibian lens regeneration

Lens regeneration among vertebrates is basically restricted to some amphibians. The most notable cases are the ones that occur in premetamorphic frogs and in adult newts. Frogs and newts regenerate their lens in very different ways. In frogs the lens is regenerated by transdifferentiation of the cornea and is limited only to a time before metamorphosis. On the other hand, regeneration in newts is mediated by transdifferentiation of the pigment epithelial cells of the dorsal iris and is possible in adult animals as well. Thus, the study of both systems could provide important information about the process. Molecular tools have been developed in frogs and recently also in newts. Thus, the process has been studied at the molecular and cellular levels. A synthesis describing both systems was long due. In this review we describe the process in both Xenopus and the newt. The known molecular mechanisms are described and compared.

Autosomal dominant congenital nuclear cataracts caused by a CRYAA gene mutation

PURPOSE

We sought to identify the genetic defect in a four-generation Chinese family with autosomal dominant congenital nuclear cataracts, examine the clinical features in detail and demonstrate the functional analysis of a candidate gene in the family.

METHODS

Family history data were recorded. Clinical and ophthalmological examinations were performed on affected and unaffected family members. All the members were genotyped with microsatellite markers at loci considered to be associated with cataracts. Two-point LOD scores were calculated using the LINKAGE program package after genotyping. A mutation was detected by dilff521229rect sequencing and verified by denaturing high-performance liquid chromatography (DHPLC). Wild-type and mutant proteins were analyzed with online softwares.

RESULTS

All affected members of this family had nuclear cataracts. Genetic analysis revealed a heterozygous previously described Arg116Cys mutation in the CRYAA gene in all of the affected members of the family but not in unaffected or 100 normal, unrelated individuals. Data generated with online software revealed that the different amino acid side chain, impact the aa116 interaction with other amino acids, thereby affecting the proteins secondary structure.

CONCLUSIONS

This study identified a mutation in the CRYAA gene causing autosomal dominant nuclear cataracts and some patients show nystagmus or small blepharophimosis clinical features. These results provide evidence that CRYAA is a pathogenic gene for congenital cataracts, congenital cataracts are a clinically and genetically heterogeneous lens condition; at the same time, demonstrates a possible mechanism of action for the mutant gene.

KSHV-encoded miRNAs target MAF to induce endothelial cell reprogramming

Kaposi sarcoma herpesvirus (KSHV) induces transcriptional reprogramming of endothelial cells. In particular, KSHV-infected lymphatic endothelial cells (LECs) show an up-regulation of genes associated with blood vessel endothelial cells (BECs). Consequently, KSHV-infected tumor cells in Kaposi sarcoma are poorly differentiated endothelial cells, expressing markers of both LECs and BECs. MicroRNAs (miRNAs) are short noncoding RNA molecules that act post-transcriptionally to negatively regulate gene expression. Here we validate expression of the KSHV-encoded miRNAs in Kaposi sarcoma lesions and demonstrate that these miRNAs contribute to viral-induced reprogramming by silencing the cellular transcription factor MAF (musculoaponeurotic fibrosarcoma oncogene homolog). MAF is expressed in LECs but not in BECs. We identify a novel role for MAF as a transcriptional repressor, preventing expression of BEC-specific genes, thereby maintaining the differentiation status of LECs. These findings demonstrate that viral miRNAs could influence the differentiation status of infected cells, and thereby contribute to KSHV-induced oncogenesis.

Pitx3 controls multiple aspects of lens development

The transcription factor Pitx3 is critical for lens formation. Deletions in the promoter of this gene cause abnormal lens development in the aphakia (ak) mouse mutant, which has only rudimentary lenses. In this study, we investigated the role of Pitx3 in lens development and differentiation. We found that reduced expression of Pitx3 leads to changes in the proliferation, differentiation and survival of lens cells. The genetic interactions between Pitx3 and Foxe3 were investigated, as these two transcription factors are expressed at the same time in lens development and their absence has similar consequences for lens development. We found no evidence that these two genes genetically interact. In general, our study shows that the abnormal phenotype of the ak lenses is not due to just one molecular pathway, rather in the absence of Pitx3 expression multiple aspects of lens development are disrupted.

NFATc1 regulates lymphatic endothelial development

NFATc1 transcription factor is critical for lineage selection in T-cell differentiation, cardiac valve morphogenesis and osteoclastogenesis. We identified a role for calcineurin-NFAT signaling in lymphatic development and patterning. NFATc1 was colocalized with lymphatic markers Prox-1, VEGFR-3 and podoplanin on cardinal vein as lymphatic endothelial cells (LEC) are specified and as they segregate into lymph sacs and mature lymphatics. In NFATc1 null mice, Prox-1, VEGFR-3 and podoplanin positive endothelial cells sprouted from the cardinal vein at E11.5, but poorly coalesced into lymph sacs. NFAT activation requires the phosphatase calcineurin. Embryos treated in utero with the calcineurin inhibitor cyclosporine-A showed cytoplasmic NFATc1, diminished podoplanin and FGFR-3 expression by the lymphatics and irregular patterning of the LEC sprouts coming off the jugular lymph sac, which suggests a role for calcineurin-NFAT signaling in lymphatic patterning. In a murine model of injury-induced lymphangiogenesis, NFATc1 was expressed on the neolymphatics induced by lung-specific overexpression of VEGF-A. Mice lacking the calcineurin Abeta regulatory subunit, with diminished nuclear NFAT, failed to respond to VEGF-A with increased lymphangiogenesis. In vitro, endogenous and VEGF-A-induced VEGFR-3 and podoplanin expression by human microvascular endothelial cells was reduced by siRNA to NFATc1, to levels comparable to reductions seen with siRNA to Prox-1. In reporter assays, NFATc1 activated lymphatic specific gene promoters. These results demonstrate the role of calcineurin-NFAT pathway in lymphangiogenesis and suggest that NFATc1 is the principle NFAT involved.

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.

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.

Identification and characterization of zebrafish ocular formation genes

To study genes that are specifically expressed in the eyes, we employed microarray and in situ hybridization analyses to identify and characterize differentially expressed ocular genes in eyeless masterblind (mbl-/-) zebrafish (Danio rerio). Among 70 differentially expressed genes in the mbl-/- mutant identified by microarray analysis, 8 down-regulated genes were characterized, including 4 eye-specific genes, opsin 1 short-wave-sensitive 1 (opn1sw1), crystallinbetaa1b (cryba1b), crystallinbetaa2b (cryba2b), and crystallingamma M2d3 (crygm2d3); 2 eye and brain genes, ATPase, H+ transporting, lysosomal, V0 subunit c (atp6v0c) and basic leucine zipper and W2 domains 1a (bzw1a); and 2 constitutive genes, heat shock protein 8 (hspa8) and ribosomal protein L7a (rpl7a). In situ hybridization experiments confirmed down-regulation of these 8 ocular formation genes in mbl-/- zebrafish and showed their ocular and dynamic temporal expression patterns during zebrafish early development. Further, an automated literature analysis of the 70 differentially expressed genes identified a sub-network of genes with known associations, either with each other or with ocular structures or development, and shows how this study contributes to the current body of knowledge.

Dual roles for Prox1 in the regulation of the chicken betaB1-crystallin promoter

PURPOSE

Lens fiber cell differentiation is marked by the onset of betaB1-crystallin expression and is controlled by the cooperative action of a set of transcription factors including Prox1, an atypical homeodomain protein. Previously, the authors reported that Prox1 directly interacts with the OL2 element found in the chicken betaB1-crystallin basal promoter to activate the expression of this gene. Here they mapped the location of activating and repressing sequences of the full-length chicken betaB1-crystallin promoter (-432/+30) in lens epithelial cells, annular pad cells, and intact lens and characterized Prox1-binding sites found in this region.

METHODS

Transfection analysis and transgenic mice were used to characterize upstream regions of the chicken betaB1-crystallin gene. DNaseI footprinting and chromatin immunoprecipitation was performed to identify Prox1-binding sites, and transfection analyses were used to characterize these sites functionally.

RESULTS

Sequences between -152 and -432 of the chicken betaB1-crystallin promoter mediated either promoter activation or repression, depending on the stage of lens differentiation tested. Two new Prox1-binding sites were found in this region that bound Prox1 more avidly than the OL2 element. However, neither binding site conferred Prox1-mediated activation on a heterologous promoter; instead, each allowed Prox1 to repress promoter function.

CONCLUSIONS

The function of the upstream region of the chicken betaB1-crystallin promoter changes depending on cellular context. These data suggest that Prox1 function as a transcriptional activator could be regulated at the DNA level based on the characteristics of the responsive elements.

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.

The lens-regenerating competence in the outer cornea and epidermis of larval Xenopus laevis is related to pax6 expression

After lentectomy, larval Xenopus laevis can regenerate a new lens by transdifferentiation of the outer cornea and pericorneal epidermis (lentogenic area). This process is promoted by retinal factor(s) accumulated into the vitreous chamber. To understand the molecular basis of the lens-regenerating competence (i.e. the capacity to respond to the retinal factor forming a new lens) in the outer cornea and epidermis, we analysed the expression of otx2, pax6, sox3, pitx3, prox1, betaB1-cry (genes all involved in lens development) by Real-time RT-PCR in the cornea and epidermis fragments dissected from donor larvae. The same fragments were also implanted into the vitreous chamber of host larvae to ascertain their lens-regenerating competence using specific anti-lens antibodies. The results demonstrate that there is a tight correlation between lens-regenerating competence and pax6 expression. In fact, (1) pax6 is the only one of the aforesaid genes to be expressed in the lentogenic area; (2) pax6 expression is absent in head epidermis outside the lentogenic area and in flank epidermis, both incapable of transdifferentiating into lens after implantation into the vitreous chamber; (3) in larvae that have undergone eye transplantation under the head or flank epidermis, pax6 re-expression was observed only in the head epidermis covering the transplanted eye. This is consistent with the fact that only the head epidermis reacquires the lens-regenerating competence after eye transplantation, forming a lens following implantation into the vitreous chamber; and (4) in larvae that have undergone removal of the eye, the epidermis covering the orbit maintained pax6 expression. This is consistent with the fact that after the eye enucleation the lentogenic area maintains the lens-regenerating competence, giving rise to a lens after implantation into the vitreous chamber. Moreover, we observed that misexpression of pax6 is sufficient to promote the acquisition of the lens-regenerating competence in flank epidermis. In fact, flank epidermis fragments dissected from pax6 RNA injected embryos could form lenses when implanted into the vitreous chamber. The data indicate for the first time that pax6 is a pivotal factor of lens-regenerating competence in the outer cornea and epidermis of larval X. laevis.

GAD isoforms exhibit distinct spatiotemporal expression patterns in the developing mouse lens: correlation with Dlx2 and Dlx5

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter of the adult nervous system and its biosynthetic enzyme glutamic acid decarboxylase (GAD) are abundantly expressed in the embryonic nervous system and are involved in the modulation of cell proliferation, migration, and differentiation. Here we describe for the first time the expression of GABA and embryonic and adult GAD isoforms in the developing mouse lens. We show that the GAD isoforms are sequentially induced with specific spatiotemporal profiles: GAD65 and embryonic GAD isoforms prevail in primary fibers, while GAD67 is the predominant GAD expressed in the postnatal secondary fibers. This pattern correlates well with the expression of Dlx2 and Dlx5, known as upstream regulators of GAD. GABA and GAD are most abundant at the tips of elongating fibers and are absent from organelle-free cells, suggesting their involvement is primarily in shaping of the cytoskeleton during fiber elongation stages.

Lens gene expression analysis reveals downregulation of the anti-apoptotic chaperone alphaA-crystallin during cavefish eye degeneration

We have conducted a survey of the expression patterns of five genes encoding three different classes of major lens proteins during eye degeneration in the blind cavefish Astyanax mexicanus. This species consists of two forms, an eyed surface-dwelling form (surface fish) and a blind cave-dwelling (cavefish) form. Cavefish form an optic primordium with a lens vesicle and optic cup. In contrast to surface fish, however, the cavefish lens does not differentiate fiber cells and undergoes massive apoptosis. The genes encoding the lens intrinsic membrane proteins MIP and MP19 and the divergent betaB1- and gammaM2-crystallins are expressed during cavefish lens development, although their levels are reduced because of a smaller lens, and the spatial distribution of their transcripts is modified because of the lack of differentiated fiber cells. In contrast, the alphaA-crystallin gene, which encodes a heat shock protein-related chaperone with antiapoptotic activity, is substantially downregulated in the developing cavefish lens. The results suggest that suppression of alphaA-crystallin antiapoptotic activity may be involved in cavefish eye degeneration.

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.

Conditional deletion of beta1-integrin from the developing lens leads to loss of the lens epithelial phenotype

Beta1-integrins are cell surface receptors that participate in sensing the cell's external environment. We used the Cre-lox system to delete beta1-integrin in all lens cells as the lens vesicle transitions into the lens. Adult mice lacking beta1-integrin in the lens are microphthalmic due to apoptosis of the lens epithelium and neonatal disintegration of the lens fibers. The first morphological alterations in beta1-integrin null lenses are seen at 16.5 dpc when the epithelium becomes disorganized and begins to upregulate the fiber cell markers beta- and gamma-crystallins, the transcription factors cMaf and Prox1 and downregulate Pax6 levels demonstrating that beta1-integrin is essential to maintain the lens epithelial phenotype. Furthermore, beta1-integrin null lens epithelial cells upregulate the expression of alpha-smooth muscle actin and nuclear Smad4 and downregulate Smad6 suggesting that beta1-integrin may brake TGFbeta family signaling leading to epithelial-mesenchymal transitions in the lens. In contrast, beta1-integrin null lens epithelial cells show increased E-cadherin immunoreactivity which supports the proposed role of beta1-integrins in mediating complete EMT in response to TGFbeta family members. Thus, beta1-integrin is required to maintain the lens epithelial phenotype and block inappropriate activation of some aspects of the lens fiber cell differentiation program.

Crystallins in the eye: Function and pathology

Crystallins are the predominant structural proteins in the lens that are evolutionarily related to stress proteins. They were first discovered outside the vertebrate eye lens by Bhat and colleagues in 1989 who found alphaB-crystallin expression in the retina, heart, skeletal muscles, skin, brain and other tissues. With the advent of microarray and proteome analysis, there is a clearer demonstration that crystallins are prominent proteins both in the normal retina and in retinal pathologies, emphasizing the importance of understanding crystallin functions outside of the lens. There are two main crystallin gene families: alpha-crystallins, and betagamma-crystallins. alpha-crystallins are molecular chaperones that prevent aberrant protein interactions. The chaperone properties of alpha-crystallin are thought to allow the lens to tolerate aging-induced deterioration of the lens proteins without showing signs of cataracts until older age. alpha-crystallins not only possess chaperone-like activity in vitro, but can also remodel and protect the cytoskeleton, inhibit apoptosis, and enhance the resistance of cells to stress. Recent advances in the field of structure-function relationships of alpha-crystallins have provided the first clues to their underlying roles in tissues outside the lens. Proteins of the betagamma-crystallin family have been suggested to affect lens development, and are also expressed in tissues outside the lens. The goal of this paper is to highlight recent work with lens epithelial cells from alphaA- and alphaB-crystallin knockout mice. The use of lens epithelial cells suggests that crystallins have important cellular functions in the lens epithelium and not just the lens fiber cells as previously thought. These studies may be directly relevant to understanding the general cellular functions of crystallins.

Isolation and characterization of a novel gene, xMADML, involved in Xenopus laevis eye development

We have identified Xenopus MADM-like (xMADML), a Xenopus laevis gene related to the murine MADM and the human NRBP genes. xMADML is expressed throughout early development and is expressed most strongly in the developing lens and more weakly in the retina and other anterior tissues. We demonstrate that disruption of xMADML translation by means of morpholino injection results in impaired retina and lens development. Reciprocal transplantation of the presumptive lens ectoderm between morpholino-injected embryos and those injected solely with a dextran lineage tracer demonstrates that xMADML is necessary in both the lens and the retina for correct development of these eye tissues. Analysis of gene expression after knockdown of xMADML revealed significant alterations in the expression of some genes, including Pax6, xSix3, Sox2, and Sox3, suggesting that xMADML plays a role in regulating gene expression during development of the eye. This investigation is the first in vivo study examining the developmental role of this novel gene and reveals an important role of xMADML in eye tissue development and differentiation.

Zebrafish foxe3: roles in ocular lens morphogenesis through interaction with pitx3

Foxe3 is a winged helix/forkhead domain transcription factor necessary for mammalian and amphibian lens development. Human FOXE3 mutations cause anterior segment dysgenesis and cataracts. The zebrafish foxe3 cDNA was PCR amplified from 24 h post-fertilization (hpf) embryo cDNA. The zebrafish foxe3 gene consists of a single exon on chromosome 8 and encodes a 422 amino acid protein. This protein possesses 44% and 67% amino acid identity with the human FOXE3 and Xenopus FoxE4 proteins, respectively. A polyclonal antiserum was generated against a bacterial fusion protein containing the Foxe3 carboxyl terminus. The purified antiserum detects zebrafish Foxe3 on immunoblots, in embryo wholemounts, and frozen tissue sections. The zebrafish Foxe3 protein is first detected in the lens at 31hpf and is restricted to the nucleated cell population, including the epithelial and elongating fiber cells. Knockdown of Foxe3 protein using an antisense morpholino results in small lenses with multilayered epithelial cells and fiber cell dysmorphogenesis. The morphants posses normal retinas, although retinal cell proteins, including rhodopsin, are abnormally expressed in the morphant lens tissue. Functional interactions between foxe3 and pitx3 during lens development were assessed by RT-PCR and comparison of Foxe3 and Pitx3 protein expression in both foxe3 and pitx3 morphants. Immunoblots and immunohistochemistry reveal Pitx3 is expressed in the foxe3 morphant lens, while Pitx3 knockdown results in the elimination of Foxe3 expression. These data demonstrate that Foxe3 is necessary for lens development in zebrafish and that foxe3 lies genetically downstream of pitx3 in a zebrafish lens development pathway.