Spatial and temporal expression of Wnt and Dickkopf genes during murine lens development

The use of shotgun label-free quantitative proteomic mass spectrometry to evaluate the inflammatory response in aqueous humor from horses with uveitis compared to ophthalmologically healthy horses

OBJECTIVE

The objective of this study was to use shotgun label-free tandem mass spectrometry (LF-MS/MS) to evaluate aqueous humor (AH) from horses with uveitis (UH) compared to ophthalmologically healthy horses (HH).

ANIMALS STUDIED

Twelve horses diagnosed with uveitis based on ophthalmic examination and six ophthalmologically healthy horses (postmortem) purchased for teaching purposes.

PROCEDURES

All horses received a complete ophthalmic examination and physical exam. Aqueous paracentesis was performed on all horses and AH total protein concentrations were measured with nanodrop (TPn) and refractometry (TPr). AH samples were analyzed with shotgun LF-MS/MS and proteomic data were compared between groups using Wilcoxon rank-sum test.

RESULTS

A total of 147 proteins were detected, 11 proteins had higher abundance in UH, and 38 proteins had lower abundance in UH. Proteins with higher abundance included apolipoprotein E, alpha-2-macroglobulin (A2M), alpha-2-HS-glycoprotein, prothrombin, fibrinogen, complement component 4 (C4), joining chain for IgA and IgM, afamin, and amine oxidase. There were positive correlations between TPn (p = .003) and TPr (p = .0001) compared to flare scores.

CONCLUSION

Differential abundance of A2M, prothrombin, fibrinogen, and C4 indicate upregulation of the complement and coagulation cascade in equine uveitis. Proinflammatory cytokines and the complement cascade have potential as therapeutic targets for equine uveitis.

Bi-allelic variants in WNT7B disrupt the development of multiple organs in humans

BACKGROUND

Pulmonary hypoplasia, Diaphragmatic anomalies, Anophthalmia/microphthalmia and Cardiac defects delineate the PDAC syndrome. We aim to identify the cause of PDAC syndrome in patients who do not carry pathogenic variants in RARB and STRA6, which have been previously associated with this disorder.

METHODS

We sequenced the exome of patients with unexplained PDAC syndrome and performed functional validation of candidate variants.

RESULTS

We identified bi-allelic variants in WNT7B in fetuses with PDAC syndrome from two unrelated families. In one family, the fetus was homozygous for the c.292C>T (p.(Arg98*)) variant whereas the fetuses from the other family were compound heterozygous for the variants c.225C>G (p.(Tyr75*)) and c.562G>A (p.(Gly188Ser)). Finally, a molecular autopsy by proxy in a consanguineous couple that lost two babies due to lung hypoplasia revealed that both parents carry the p.(Arg98*) variant. Using a WNT signalling canonical luciferase assay, we demonstrated that the identified variants are deleterious. In addition, we found that wnt7bb mutant zebrafish display a defect of the swimbladder, an air-filled organ that is a structural homolog of the mammalian lung, suggesting that the function of WNT7B has been conserved during evolution for the development of these structures.

CONCLUSION

Our findings indicate that defective WNT7B function underlies a form of lung hypoplasia that is associated with the PDAC syndrome, and provide evidence for involvement of the WNT-β-catenin pathway in human lung, tracheal, ocular, cardiac, and renal development.

Multiple roles of Pax6 in postnatal cornea development

Mammalian corneal development is a multistep process, including formation of the corneal epithelium (CE), endothelium and stroma during embryogenesis, followed by postnatal stratification of the epithelial layers and continuous renewal of the epithelium to replace the outermost corneal cells. Here, we employed the Cre-loxP system to conditionally deplete Pax6 proteins in two domains of ocular cells, i.e., the ocular surface epithelium (cornea, limbus and conjunctiva) (OSE) or postnatal CE via K14-cre or Aldh3-cre, respectively. Earlier and broader inactivation of Pax6 in the OSE resulted in thickened OSE with CE and limbal cells adopting the conjunctival keratin expression pattern. More restricted depletion of Pax6 in postnatal CE resulted in an abnormal cornea marked by reduced epithelial thickness despite increased epithelial cell proliferation. Immunofluorescence studies revealed loss of intermediate filament Cytokeratin 12 and diffused expression of adherens junction components, together with reduced tight junction protein, Zonula occludens-1. Furthermore, the expression of Cytokeratin 14, a basal cell marker in apical layers, indicates impaired differentiation of CE cells. Collectively, our data demonstrate that Pax6 is essential for maintaining proper differentiation and strong intercellular adhesion in postnatal CE cells, whereas limbal Pax6 is required to prevent the outgrowth of conjunctival cells to the cornea.

Aniridia-related keratopathy relevant cell signaling pathways in human fetal corneas

We aimed to study aniridia-related keratopathy (ARK) relevant cell signaling pathways [Notch1, Wnt/β-catenin, Sonic hedgehog (SHH) and mTOR] in normal human fetal corneas compared with normal human adult corneas and ARK corneas. We found that fetal corneas at 20 weeks of gestation (wg) and normal adult corneas showed similar staining patterns for Notch1; however 10–11 wg fetal corneas showed increased presence of Notch1. Numb and Dlk1 had an enhanced presence in the fetal corneas compared with the adult corneas. Fetal corneas showed stronger immunolabeling with antibodies against β-catenin, Wnt5a, Wnt7a, Gli1, Hes1, p-rpS6, and mTOR when compared with the adult corneas. Gene expression of Notch1, Wnt5A, Wnt7A, β-catenin, Hes1, mTOR, and rps6 was higher in the 9–12 wg fetal corneas compared with adult corneas. The cell signaling pathway differences found between human fetal and adult corneas were similar to those previously found in ARK corneas with the exception of Notch1. Analogous profiles of cell signaling pathway activation between human fetal corneas and ARK corneas suggests that there is a less differentiated host milieu in ARK.

Changes in DNA methylation hallmark alterations in chromatin accessibility and gene expression for eye lens differentiation

Background

Methylation at cytosines (mCG) is a well-known regulator of gene expression, but its requirements for cellular differentiation have yet to be fully elucidated. A well-studied cellular differentiation model system is the eye lens, consisting of a single anterior layer of epithelial cells that migrate laterally and differentiate into a core of fiber cells. Here, we explore the genome-wide relationships between mCG methylation, chromatin accessibility and gene expression during differentiation of eye lens epithelial cells into fiber cells.

Results

Whole genome bisulfite sequencing identified 7621 genomic loci exhibiting significant differences in mCG levels between lens epithelial and fiber cells. Changes in mCG levels were inversely correlated with the differentiation state-specific expression of 1285 genes preferentially expressed in either lens fiber or lens epithelial cells (Pearson correlation r = − 0.37, p < 1 × 10^–42). mCG levels were inversely correlated with chromatin accessibility determined by assay for transposase-accessible sequencing (ATAC-seq) (Pearson correlation r = − 0.86, p < 1 × 10^–300). Many of the genes exhibiting altered regions of DNA methylation, chromatin accessibility and gene expression levels in fiber cells relative to epithelial cells are associated with lens fiber cell structure, homeostasis and transparency. These include lens crystallins (CRYBA4, CRYBB1, CRYGN, CRYBB2), lens beaded filament proteins (BFSP1, BFSP2), transcription factors (HSF4, SOX2, HIF1A), and Notch signaling pathway members (NOTCH1, NOTCH2, HEY1, HES5). Analysis of regions exhibiting cell-type specific alterations in DNA methylation revealed an overrepresentation of consensus sequences of multiple transcription factors known to play key roles in lens cell differentiation including HIF1A, SOX2, and the MAF family of transcription factors.

Conclusions

Collectively, these results link DNA methylation with control of chromatin accessibility and gene expression changes required for eye lens differentiation. The results also point to a role for DNA methylation in the regulation of transcription factors previously identified to be important for lens cell differentiation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-022-00440-z.

Lens regeneration: scientific discoveries and clinical possibilities

In the process of exploring new methods for cataract treatment, lens regeneration is an ideal strategy for effectively restoring accommodative vision and avoiding postoperative complications and has great clinical potential. Lens regeneration, which is not a simple repetition of lens development, depends on the complex regulatory network comprising the FGF, BMP/TGF-β, Notch, and Wnt signaling pathways. Current research mainly focuses on in situ and in vitro lens regeneration. On the one hand, the possibility of the autologous stem cell in situ regeneration of functional lenses has been confirmed; on the other hand, both embryonic stem cells and induced pluripotent stem cells have been induced into lentoid bodies in vitro which are similar to the natural lens to a certain extent. This article will briefly summarize the regulatory mechanisms of lens development, describe the recent progress of lens regeneration, explore the key molecular signaling pathways, and, more importantly, discuss the prospects and challenges of their clinical applications to provide reference for clinical transformations.

Corneal development: Role of the periocular mesenchyme and bi-directional signaling

The cornea is a highly specialized transparent tissue located at the anterior most surface of the eye. It consists of three main layers, the outer stratified squamous epithelium, the inner endothelium, and the intermediate stroma. Formation of these layers during development involves a complex interaction between ectodermal-derived structures, such as the overlying head ectoderm with the periocular mesenchyme (POM), the latter of which is comprised of neural crest cells (NCC) and mesoderm-derived progenitor cells. Regulation of corneal epithelial development, including both epithelial cell fate and stratification, has been shown to depend on numerous bi-directional mesenchymal-epithelial signaling pathways. In this review we pay particular attention to the genes and signaling pathways that involve the POM.

MS/MS in silico subtraction-based proteomic profiling as an approach to facilitate disease gene discovery: application to lens development and cataract

While the bioinformatics resource-tool iSyTE (integrated Systems Tool for Eye gene discovery) effectively identifies human cataract-associated genes, it is currently based on just transcriptome data, and thus, it is necessary to include protein-level information to gain greater confidence in gene prioritization. Here, we expand iSyTE through development of a novel proteome-based resource on the lens and demonstrate its utility in cataract gene discovery. We applied high-throughput tandem mass spectrometry (MS/MS) to generate a global protein expression profile of mouse lens at embryonic day (E)14.5, which identified 2371 lens-expressed proteins. A major challenge of high-throughput expression profiling is identification of high-priority candidates among the thousands of expressed proteins. To address this problem, we generated new MS/MS proteome data on mouse whole embryonic body (WB). WB proteome was then used as a reference dataset for performing "in silico WB-subtraction" comparative analysis with the lens proteome, which effectively identified 422 proteins with lens-enriched expression at ≥ 2.5 average spectral counts, ≥ 2.0 fold enrichment (FDR < 0.01) cut-off. These top 20% candidates represent a rich pool of high-priority proteins in the lens including known human cataract-linked genes and many new potential regulators of lens development and homeostasis. This rich information is made publicly accessible through iSyTE (https://research.bioinformatics.udel.edu/iSyTE/), which enables user-friendly visualization of promising candidates, thus making iSyTE a comprehensive tool for cataract gene discovery.

Molecular characterization of the human lens epithelium-derived cell line SRA01/04

Cataract-associated gene discovery in human and animal models have informed on key aspects of human lens development, homeostasis and pathology. Additionally, in vitro models such as the culture of permanent human lens epithelium-derived cell lines (LECs) have also been utilized to understand the molecular biology of lens cells. However, these resources remain uncharacterized, specifically regarding their global gene expression and suitability to model lens cell biology. Therefore, we sought to molecularly characterize gene expression in the human LEC, SRA01/04, which is commonly used in lens studies. We first performed short tandem repeat (STR) analysis and validated SRA01/04 LEC for its human origin, as recommended by the eye research community. Next, we used Illumina HumanHT-12 v3.0 Expression BeadChip arrays to gain insights into the global gene expression profile of SRA01/04. Comparative analysis of SRA01/04 microarray data was performed using other resources such as the lens expression database iSyTE (integrated Systems Tool for Eye gene discovery), the cataract gene database Cat-Map and the published lens literature. This analysis showed that SRA01/04 significantly expresses >40% of the top iSyTE lens-enriched genes (313 out of 749) across different developmental stages. Further, SRA01/04 also significantly expresses ~53% (168 out of 318) of cataract-associated genes in Cat-Map. We also performed comparative gene expression analysis between SRA01/04 cells and the previously validated mouse LEC 21EM15. To gain insight into whether SRA01/04 reflects epithelial or fiber cell characteristics, we compared its gene expression profile to previously reported differentially expressed genes in isolated mouse lens epithelial and fiber cells. This analysis suggests that SRA01/04 has reduced expression of several fiber cell-enriched genes. In agreement with these findings, cell culture analysis demonstrates that SRA01/04 has reduced potential to initiate spontaneous lentoid body formation compared to 21EM15 cells. Next, to independently validate SRA01/04 microarray gene expression, we subjected several candidate genes to RT-PCR and RT-qPCR assays. This analysis demonstrates that SRA01/04 supports expression of many key genes associated with lens development and cataract, including CRYAB, CRYBB2, CRYGS, DKK3, EPHA2, ETV5, GJA1, HSPB1, INPPL1, ITGB1, PAX6, PVRL3, SFRP1, SPARC, TDRD7, and VIM, among others, and therefore can be relevant for understanding the mechanistic basis of these factors. At the same time, SRA01/04 cells do not exhibit robust expression of several genes known to be important to lens biology and cataract such as ALDH1A1, COL4A6, CP, CRYBA4, FOXE3, HMX1, HSF4, MAF, MEIS1, PITX3, PRX, SIX3, and TRPM3, among many others. Therefore, the present study offers a rich transcript-level resource for case-by-case evaluation of the potential advantages and limitations of SRA01/04 cells prior to their use in downstream investigations. In sum, these data show that the human LEC, SRA01/04, exhibits lens epithelial cell-like character reflected in the expression of several lens-enriched and cataract-associated genes, and therefore can be considered as a useful in vitro resource when combined with in vivo studies to gain insight into specific aspects of human lens epithelial cells.

Diverse Evolutionary Origins and Mechanisms of Lens Regeneration

In this review, we compare and contrast the three different forms of vertebrate lens regeneration: Wolffian lens regeneration, cornea-lens regeneration, and lens regeneration from lens epithelial cells. An examination of the diverse cellular origins of these lenses, their unique phylogenetic distribution, and the underlying molecular mechanisms, suggests that these different forms of lens regeneration evolved independently and utilize neither conserved nor convergent mechanisms to regulate these processes.

Spatiotemporal dynamics of canonical Wnt signaling during embryonic eye development and posterior capsular opacification (PCO)

The appropriate spatial and temporal regulation of canonical Wnt signaling is vital for eye development. However, the literature often conflicts on the distribution of canonical Wnt signaling in the eye. Here, using a sensitive mouse transgenic reporter line, we report a detailed re-evaluation of the spatiotemporal dynamics of canonical Wnt signaling in the developing eye. Canonical Wnt activity was dynamic in the optic vesicle and later in the retina, while it was absent from the ectodermal precursors of the lens and corneal epithelium. However, later in corneal development, canonical Wnt reporter activity was detected in corneal stroma and endothelium precursors as they form from the neural crest, although this was lost around birth. Interestingly, while no canonical Wnt signaling was detected in the corneal limbus or basal cells at any developmental stage, it was robust in adult corneal wing and squamous epithelial cells. While canonical Wnt reporter activity was also absent from the postnatal lens, upon lens injury intended to model cataract surgery, it upregulated within 12 h in remnant lens epithelial cells, and co-localized with alpha smooth muscle actin in fibrotic lens epithelial cells from 48 h post-surgery onward. This pattern correlated with downregulation of the inhibitor of canonical Wnt signaling, Dkk3. These data demonstrate that canonical Wnt signaling is dynamic within the developing eye and upregulates in lens epithelial cells in response to lens injury. As canonical Wnt signaling can collaborate with TGFβ to drive fibrosis in other systems, these data offer the first evidence in a lens-injury model that canonical Wnt may synergize with TGFβ signaling to drive fibrotic posterior capsular opacification (PCO).

Characterization of Lgr6+ Cells as an Enriched Population of Hair Cell Progenitors Compared to Lgr5+ Cells for Hair Cell Generation in the Neonatal Mouse Cochlea

Hair cell (HC) loss is irreversible because only very limited HC regeneration has been observed in the adult mammalian cochlea. Wnt/β-catenin signaling regulates prosensory cell proliferation and differentiation during cochlear development, and Wnt activation promotes the proliferation of Lgr5+ cochlear HC progenitors in newborn mice. Similar to Lgr5, Lgr6 is also a Wnt downstream target gene. Lgr6 is reported to be present in adult stem cells in the skin, nail, tongue, lung, and mammary gland, and this protein is very important for adult stem cell maintenance in rapidly proliferating organs. Our previous studies showed that Lgr6+ cells are a subpopulation of Lgr5+ progenitor cells and that both Lgr6+ and Lgr5+ progenitors can generate Myosin7a+ HCs in vitro. Thus we hypothesized that Lgr6+ cells are an enriched population of cochlear progenitor cells. However, the detailed distinctions between the Lgr5+ and Lgr6+ progenitors are unclear. Here, we systematically compared the proliferation, HC differentiation, and detailed transcriptome expression profiles of these two progenitor populations. We found that the same number of isolated Lgr6+ progenitors generated significantly more Myosin7a+ HCs compared to Lgr5+ progenitors; however, Lgr5+ progenitors formed more epithelial colonies and more spheres than Lgr6+ progenitors in vitro. Using RNA-Seq, we compared the transcriptome differences between Lgr5+ and Lgr6+ progenitors and identified a list of significantly differential expressed genes that might regulate the proliferation and differentiation of these HC progenitors, including 4 cell cycle genes, 9 cell signaling pathway genes, and 54 transcription factors. In conclusion, we demonstrate that Lgr6+ progenitors are an enriched population of inner ear progenitors that generate more HCs compared to Lgr5+ progenitors in the newborn mouse cochlea, and the our research provides a series of genes that might regulate the proliferation of progenitors and HC generation.

The Transcription Factor Foxg1 Promotes Optic Fissure Closure in the Mouse by Suppressing Wnt8b in the Nasal Optic Stalk

During vertebrate eye morphogenesis, a transient fissure forms at its inferior part, known as the optic fissure. This will gradually close, giving rise to a healthy, spherical optic cup. Failure of the optic fissure to close gives rise to an ocular disorder known as coloboma. During this developmental process, Foxg1 is expressed in the optic neuroepithelium, with highest levels of expression in the nasal optic stalk. Foxg1-/- mutant mice have microphthalmic eyes with a large ventral coloboma. We found Wnt8b expression upregulated in the Foxg1-/- optic stalk and hypothesized that, similar to what is observed in telencephalic development, Foxg1 directs development of the optic neuroepithelium through transcriptional suppression of Wnt8b To test this, we generated Foxg1-/-;Wnt8b-/- double mutants of either sex and found that the morphology of the optic cup and stalk and the closure of the optic fissure were substantially rescued in these embryos. This rescue correlates with restored Pax2 expression in the anterior tip of the optic fissure. In addition, although we do not find evidence implicating altered proliferation in the rescue, we observe a significant increase in apoptotic cell density in Foxg1-/-;Wnt8b-/- double mutants compared with the Foxg1-/- single mutant. Upregulation of Wnt/β-catenin target molecules in the optic cup and stalk may underlie the molecular and morphological defects in the Foxg1-/- mutant. Our results show that proper optic fissure closure relies on Wnt8b suppression by Foxg1 in the nasal optic stalk to maintain balanced apoptosis and Pax2 expression in the nasal and temporal edges of the fissure.SIGNIFICANCE STATEMENT Coloboma is an ocular disorder that may result in a loss of visual acuity and accounts for ∼10% of childhood blindness. It results from errors in the sealing of the optic fissure (OF), a transient structure at the bottom of the eye. Here, we investigate the colobomatous phenotype of the Foxg1-/- mutant mouse. We identify upregulated expression of Wnt8b in the optic stalk of Foxg1-/- mutants before OF closure initiates. Foxg1-/-;Wnt8b-/- double mutants show a substantial rescue of the Foxg1-/- coloboma phenotype, which correlates with a rescue in molecular and cellular defects of Foxg1-/- mutants. Our results unravel a new role of Foxg1 in promoting OF closure providing additional knowledge about the molecules and cellular mechanisms underlying coloboma formation.

The lens growth process

The factors that regulate the size of organs to ensure that they fit within an organism are not well understood. A simple organ, the ocular lens serves as a useful model with which to tackle this problem. In many systems, considerable variance in the organ growth process is tolerable. This is almost certainly not the case in the lens, which in addition to fitting comfortably within the eyeball, must also be of the correct size and shape to focus light sharply onto the retina. Furthermore, the lens does not perform its optical function in isolation. Its growth, which continues throughout life, must therefore be coordinated with that of other tissues in the optical train. Here, we review the lens growth process in detail, from pioneering clinical investigations in the late nineteenth century to insights gleaned more recently in the course of cell and molecular studies. During embryonic development, the lens forms from an invagination of surface ectoderm. Consequently, the progenitor cell population is located at its surface and differentiated cells are confined to the interior. The interactions that regulate cell fate thus occur within the obligate ellipsoidal geometry of the lens. In this context, mathematical models are particularly appropriate tools with which to examine the growth process. In addition to identifying key growth determinants, such models constitute a framework for integrating cell biological and optical data, helping clarify the relationship between gene expression in the lens and image quality at the retinal plane.

Identification of host proteins, Spata3 and Dkk2, interacting with Toxoplasma gondii micronemal protein MIC3

As an obligate intracellular protozoan, Toxoplasma gondii is a successful pathogen infecting a variety of animals, including humans. As an adhesin involving in host invasion, the micronemal protein MIC3 plays important roles in host cell attachment, as well as modulation of host EGFR signaling cascade. However, the specific host proteins that interact with MIC3 are unknown and the identification of such proteins will increase our understanding of how MIC3 exerts its functions. This study was designed to identify host proteins interacting with MIC3 by yeast two-hybrid screens. Using MIC3 as bait, a library expressing mouse proteins was screened, uncovering eight mouse proteins that showed positive interactions with MIC3. Two of which, spermatogenesis-associated protein 3 (Spata3) and dickkopf-related protein 2 (Dkk2), were further confirmed to interact with MIC3 by additional protein-protein interaction tests. The results also revealed that the tandem repeat EGF domains of MIC3 were critical in mediating the interactions with the identified host proteins. This is the first study to show that MIC3 interacts with host proteins that are involved in reproduction, growth, and development. The results will provide a clearer understanding of the functions of adhesion-associated micronemal proteins in T. gondii.

Wnt/β-catenin signaling modulates corneal epithelium stratification via inhibition of Bmp4 during mouse development

The development of organs with an epithelial parenchyma relies on reciprocal mesenchymal-epithelial communication. Mouse corneal epithelium stratification is the consequence of a coordinated developmental process based on mesenchymal-epithelial interactions. The molecular mechanism underlying these interactions remains unclear. The Wnt/β-catenin signaling pathway is involved in fundamental aspects of development through the regulation of various growth factors. Here, we show that conditional ablation of either β-catenin (Ctnnb1(cKO)) or co-receptors Lrp5/6 (Lrp5/6(cKO)) in corneal stromal cells results in precocious stratification of the corneal epithelium. By contrast, ectopic expression of a murine Ctnnb1 gain-of-function mutant (Ctnnb1(cGOF)) retards corneal epithelium stratification. We also discovered that Bmp4 is upregulated in the absence of β-catenin in keratocytes, which further triggers ERK1/2 (Mapk3/1) and Smad1/5 phosphorylation and enhances transcription factor p63 (Trp63) expression in mouse corneal basal epithelial cells and in a human corneal epithelial cell line (HTCE). Interestingly, mouse neonates given a subconjunctival BMP4 injection displayed a phenotype resembling that of Ctnnb1(cKO). Conditional ablation of Bmp4 eradicates the phenotype produced in Ctnnb1(cKO) mice. Furthermore, ChIP and promoter-luciferase assays show that β-catenin binds to and suppresses Bmp4 promoter activity. These data support the concept that cross-talk between the Wnt/β-catenin/Bmp4 axis (in the stromal mesenchyme) and Bmp4/p63 signaling (in the epithelium) plays a pivotal role in epithelial stratification during corneal morphogenesis.

Differential binding of Lef1 and Msx1/2 transcription factors to Dkk1 CNEs correlates with reporter gene expression in vivo

Besides the active Wnt signalling itself, the extracellular inhibition by Dkk1 is important for various embryonic developmental processes, such as optic vesicle differentiation and facial outgrowth. Although a feedback crosstalk of the active Wnt/β-catenin signaling and Dkk1 regulation has been suggested, the control of Dkk1 transcription by the Tcf/Lef1 mediated Wnt signalling and its connection to additional signalling factors has not been elucidated in vivo. Here, we used a combination of transgenic mouse approaches and biochemical analyses to unravel the direct Dkk1 transcriptional regulation via Tcf/Lefs. By using site directed mutagenesis, we tested several conserved Tcf/Lef1 binding sites within Dkk1 conserved non-coding elements (CNEs) and found that these are required for tissue specific reporter expression. In addition a conserved Msx1/2 binding site is required for retinal reporter expression and Msx2 but not Msx1 binds its conserved binding site within CNE195 in the optic cups. Within craniofacial expression domains, Lef1 interferes with Dkk1 directly via two conserved Tcf/Lef1 binding sites in the craniofacial enhancer CNE114, both of which are required for the general craniofacial Dkk1 reporter activation. Furthermore, these Tcf/Lef1 sites are commonly bound in the whisker hair bud mesenchyme but specifically Tcf/Lef1 (no. 2) is required for mandibular activation and repression of maxillar Dkk1 activation. Lastly, we tested the Tcf/Lef1 binding capacities of the Dkk1 promoter and found that although Lef1 binds the Dkk1 promoter, these sites are not sufficient for tissue specific Dkk1 activation. Together, we here present the importance of conserved Tcf/Lef1 and Msx1/2 sites that are required for differential Dkk1 transcriptional reporter activation in vivo. This requirement directly correlates with Lef1 and Msx1/2 interaction with these genomic loci.

Molecular characterization of mouse lens epithelial cell lines and their suitability to study RNA granules and cataract associated genes

The discovery of cytosolic RNA granule (RG) component proteins associated with human cataract has initiated investigations on post-transcriptional mechanisms of gene expression control in the lens. Application of established mouse lens epithelial cell lines (LECs) can provide rapid insights on RG function in lens cells, especially because mouse mutants in several RG components are not available. However, although these LECs represent potential reagents for such analyses, they are uncharacterized for lens gene expression or RG formation. Therefore, a detailed molecular and cellular characterization of three permanent mouse LECs 17EM15, 21EM15 and αTN4 is performed in this study. Comparative analysis between microarray gene expression datasets on LEC 21EM15 and iSyTE lens tissue demonstrates that 30% of top 200 iSyTE identified lens-enriched genes are expressed in these cells. Majority of these candidates are independently validated to either have lens expression, function or linkage to cataract. Moreover, analysis of microarray data with genes described in Cat-Map, an online database of cataract associated genes and loci, demonstrates that 131 genes linked to cataract loci are expressed in 21EM15 cells. Furthermore, gene expression in LECs is compared to isolated lens epithelium or fiber cells by qRT-PCR and by comparative analyses with publically available epithelium or fiber-specific microarray and RNA-seq (sequencing) datasets. Expression of select candidate genes was validated by regular and real-time quantitative RT-PCR. Expression of lens epithelium-enriched genes Foxe3, Pax6, Anxa4 and Mcm4 is up-regulated in LEC lines, compared to isolated lens fiber cells. Moreover, similar to isolated lens epithelium, all three LECs exhibit down-regulation of fiber cell-expressed genes Crybb1, Mip and Prox1 when compared to fiber cells. These data indicate that the LEC lines exhibit greater similarity to lens epithelium than to fiber cells. Compared to non-lens cell line NIH3T3, LECs exhibit significantly enriched expression of transcription factors with important function in the lens, namely Pax6, Foxe3 and Prox1. In addition to these genes, all three LECs also express key lens- and cataract-associated genes, namely Dkk3, Epha2, Hsf4, Jag1, Mab21l1, Meis1, Pknox1, Pou2f1, Sfrp1, Sparc, Tdrd7 and Trpm3. Additionally, 21EM15 microarrays indicate expression of Chmp4b, Cryab and Tcfap2a among others important genes. Immunostaining with makers for Processing bodies (P-bodies) and Stress granules (SGs) demonstrates that these classes of RGs are robustly expressed in all three LECs. Moreover, under conditions of stress, 17EM15 and αTN4 exhibit significantly higher numbers of P-bodies and SGs compared to NIH3T3 cells. In sum, these data indicate that mouse LECs 21EM15, 17EM15 and αTN4 express key lens or cataract genes, are similar to lens epithelium than fiber cells, and exhibit high levels of P-bodies and SGs, indicating their suitability for investigating gene expression control and RG function in lens-derived cells.

Interactions between lens epithelial and fiber cells reveal an intrinsic self-assembly mechanism

How tissues and organs develop and maintain their characteristic three-dimensional cellular architecture is often a poorly understood part of their developmental program; yet, as is clearly the case for the eye lens, precise regulation of these features can be critical for function. During lens morphogenesis cells become organized into a polarized, spheroidal structure with a monolayer of epithelial cells overlying the apical tips of elongated fiber cells. Epithelial cells proliferate and progeny that shift below the lens equator differentiate into new fibers that are progressively added to the fiber mass. It is now known that FGF induces epithelial to fiber differentiation; however, it is not fully understood how these two forms of cells assemble into their characteristic polarized arrangement. Here we show that in FGF-treated epithelial explants, elongating fibers become polarized/oriented towards islands of epithelial cells and mimic their polarized arrangement in vivo. Epithelial explants secrete Wnt5 into the culture medium and we show that Wnt5 can promote directed behavior of lens cells. We also show that these explants replicate aspects of the Notch/Jagged signaling activity that has been shown to regulate proliferation of epithelial cells in vivo. Thus, our in vitro study identifies a novel mechanism, intrinsic to the two forms of lens cells, that facilitates self-assembly into the polarized arrangement characteristic of the lens in vivo. In this way the lens, with its relatively simple cellular composition, serves as a useful model to highlight the importance of such intrinsic self-assembly mechanisms in tissue developmental and regenerative processes.

DKK3 is a potential tumor suppressor gene in papillary thyroid carcinoma

The expression of the Dickkopf homolog 3 (DKK3) gene is downregulated in some human cancers, suggesting a possible tumor suppressor role of this gene. The role and regulation of DKK3 in thyroid cancer have not been examined. In this study, we explored the relationship of promoter methylation with the inactivation of DKK3 and tumor behaviors in papillary thyroid carcinoma (PTC). We used methylation-specific PCR and RT-PCR to examine the promoter methylation and expression of DKK3 and tumor characteristics. We found mRNA expression of DKK3 in 44.9% of the PTC tissue samples vs 100% of the matched normal thyroid tissue samples (P<0.01). In contrast, an opposite distribution pattern of DKK3 gene methylation was observed; specifically, 38.8% of the PTC tissue samples vs 0% of the matched normal thyroid tissue samples harbored DKK3 methylation. An inverse correlation between the promoter methylation and mRNA expression of DKK3 in PTC tissue samples was also observed. Moreover, we also found an inverse correlation between DKK3 expression and some aggressive pathological characteristics of PTC, including high TNM stages and lymph node metastasis, but a positive correlation between DKK3 promoter hypermethylation and pathological aggressiveness of the tumor. Treatment of the PTC cell line TPC-1 with the demethylating agent 5-azaC reduced DKK3 promoter methylation and enhanced its expression, establishing functionally the impact of DKK3 methylation on its expression. Our data thus for the first time demonstrate that the DKK3 gene is a potential tumor suppressor gene in thyroid cancer and that aberrant promoter methylation is an important mechanism for its downregulation, which may play a role in the tumorigenesis and aggressiveness of PTC.

Cell signaling pathways in vertebrate lens regeneration

Certain vertebrates are capable of regenerating parts of the eye, including the lens. Depending on the species, two principal forms of in vivo lens regeneration have been described wherein the new lens arises from either the pigmented epithelium of the dorsal iris or the cornea epithelium. These forms of lens regeneration are triggered by retinal factors present in the eye. Studies have begun to illuminate the nature of the signals that support lens regeneration. This review describes evidence for the involvement of specific signaling pathways in lens regeneration, including the FGF, retinoic acid, TGF-beta, Wnt, and Hedgehog pathways.

Wnt regulation of planar cell polarity (PCP)

Planar cell polarity (PCP), a process controlling coordinated, uniformly polarized cellular behaviors in a field of cells, has been identified to be critically required for many fundamental developmental processes. However, a global directional cue that establishes PCP in a three-dimensional tissue or organ with respect to the body axes remains elusive. In vertebrate, while Wnt-secreted signaling molecules have been implicated in regulating PCP in a β-catenin-independent manner, whether they function permissively or act as a global cue to convey directional information is not clearly defined. In addition, the underlying molecular mechanism by which Wnt signal is transduced to core PCP proteins is largely unknown. In this chapter, I review the roles of Wnt signaling in regulating PCP during vertebrate development and update our knowledge of its regulatory mechanism.

Wnt/beta-catenin signaling in hepatic organogenesis

Wnt/beta-catenin signaling has come to the forefront of liver biology in recent years. This pathway regulates key pathophysiological events inherent to the liver including development, regeneration and cancer, by dictating several biological processes such as proliferation, apoptosis, differentiation, adhesion, zonation and metabolism in various cells of the liver. This review will examine the studies that have uncovered the relevant roles of Wnt/beta-catenin signaling during the process of liver development. We will discuss the potential roles of Wnt/beta-catenin signaling during the phases of development, including competence, hepatic induction, expansion and morphogenesis. In addition, we will discuss the role of negative and positive regulation of this pathway and how the temporal expression of Wnt/beta-catenin can direct key processes during hepatic development. We will also identify some of the major deficits in the current understanding of the role of Wnt/beta-catenin signaling in liver development in order to provide a perspective for future studies. Thus, this review will provide a contextual overview of the role of Wnt/beta-catenin signaling during hepatic organogenesis.

Wnt signaling in eye organogenesis

The vertebrate eye consists of multiple tissues with distinct embryonic origins. To ensure formation of the eye as a functional organ, development of ocular tissues must be precisely coordinated. Besides intrinsic regulators, several extracellular pathways have been shown to participate in controlling critical steps during eye development. Many components of Wnt/Frizzled signaling pathways are expressed in developing ocular tissues, and substantial progress has been made in the past few years in understanding their function during vertebrate eye development. Here, I summarize recent work using functional experiments to elucidate the roles of Wnt/Frizzled pathways during development of ocular tissues in different vertebrates.

BAC-Dkk3-EGFP transgenic mouse: an in vivo analytical tool for Dkk3 expression

Dickkopf (DKK) family proteins are secreted modulators of the Wnt signaling pathway and are capable of regulating the development of many organs and tissues. We previously identified Dkk3 to be a molecule predominantly expressed in the mouse embryonic retina. However, which cell expresses Dkk3 in the developing and mature mouse retina remains to be elucidated. To examine the precise expression of the Dkk3 protein, we generated BAC-Dkk3-EGFP transgenic mice that express EGFP integrated into the Dkk3 gene in a BAC plasmid. Expression analysis using the BAC-Dkk3-EGFP transgenic mice revealed that Dkk3 is expressed in retinal progenitor cells (RPCs) at embryonic stages and in Müller glial cells in the adult retina. Since Müller glial cells may play a potential role in retinal regeneration, BAC-Dkk3-EGFP mice could be useful for retinal regeneration studies.

Transdifferentiation from cornea to lens in Xenopus laevis depends on BMP signalling and involves upregulation of Wnt signalling

Background

Surgical removal of the lens from larval Xenopus laevis results in a rapid transdifferention of central corneal cells to form a new lens. The trigger for this process is understood to be an induction event arising from the unprecedented exposure of the cornea to the vitreous humour that occurs following lens removal. The molecular identity of this trigger is unknown.

Results

Here, we have used a functional transgenic approach to show that BMP signalling is required for lens regeneration and a microarray approach to identify genes that are upregulated specifically during this process. Analysis of the array data strongly implicates Wnt signalling and the Pitx family of transcription factors in the process of cornea to lens transdifferentiation. Our analysis also captured several genes associated with congenital cataract in humans. Pluripotency genes, in contrast, were not upregulated, supporting the idea that corneal cells transdifferentiate without returning to a stem cell state. Several genes from the array were expressed in the forming lens during embryogenesis. One of these, Nipsnap1, is a known direct target of BMP signalling.

Conclusions

Our results strongly implicate the developmental Wnt and BMP signalling pathways in the process of cornea to lens transdifferentiation (CLT) in Xenopus, and suggest direct transdifferentiation between these two anterior eye tissues.

Planar cell polarity in the mammalian eye lens

The major role of the eye lens is to transmit and focus images onto the retina. For this function, the lens needs to develop and maintain the correct shape, notably, the precise curvature and high-level order and organization of its elements. The lens is mainly comprised of highly elongated fiber cells with hexagonal cross-sectional profiles that facilitate regular packing. Collectively, they form concentrically arranged layers around the anterior-posterior polar axis, and their convex curvature contributes to the spheroidal shape of the lens. Although the lens has been a popular system for developmental studies, little is known about the mechanism(s) that underlies the development of its exquisite three-dimensional cellular architecture. In this review, we will describe our recent work, which shows how planar cell polarity (PCP) operates in lens and contributes to its morphogenesis. We believe that the lens will be a useful model system to study PCP in general and gain insights into mechanisms that generate high-level cellular order during development.

Understanding the role of growth factors in embryonic development: insights from the lens

Growth factors play key roles in influencing cell fate and behaviour during development. The epithelial cells and fibre cells that arise from the lens vesicle during lens morphogenesis are bathed by aqueous and vitreous, respectively. Vitreous has been shown to generate a high level of fibroblast growth factor (FGF) signalling that is required for secondary lens fibre differentiation. However, studies also show that FGF signalling is not sufficient and roles have been identified for transforming growth factor-β and Wnt/Frizzled families in regulating aspects of fibre differentiation. In the case of the epithelium, key roles for Wnt/β-catenin and Notch signalling have been demonstrated in embryonic development, but it is not known if other factors are required for its formation and maintenance. This review provides an overview of current knowledge about growth factor regulation of differentiation and maintenance of lens cells. It also highlights areas that warrant future study.

The Dkk1 dose is critical for eye development

During mammalian ocular development, several signaling pathways control the spatiotemporal highly defined realization of the three-dimensional eye architecture. Given the complexity of these inductive signals, the developing eye is a sensitive organ for several diseases. In this study, we investigated a Dkk1+/- haploinsufficiency during eye development, resulting in coloboma and anterior eye defects, two common developmental eye disorders. Dkk1 impacts eye development from a defined developmental time point on, and is critical for lens separation from the surface ectoderm via β-catenin mediated Pdgfrα and E-cadherin expression. Dkk1 does not impact the dorso ventral retina patterning in general but is critical for Shh dependent Pax2 extension into the midline region. The described results also indicate that the retinal Dkk1 dose is critical for important steps during eye development, such as optic fissure closure and cornea formation. Further analysis of the relationship between Dkk1 and Shh signaling revealed that Dkk1 and Shh coordinatively control anterior head formation and eye induction. During eye development itself, retinal Dkk1 activation is depending on cilia mediated Gli3 regulation. Therefore, our data essentially improve the knowledge of coloboma and anterior eye defects, which are common human eye developmental defects.

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.

Secreted frizzled-related protein 1 (SFRP1) is highly upregulated in keratoconus epithelium: a novel finding highlighting a new potential focus for keratoconus research and treatment

PURPOSE

To investigate the expression of Wnt signalling pathway genes in keratoconic (KC) epithelium.

METHODS

RNA was extracted from the epithelium of four KC patients undergoing corneal transplantation and five age-matched controls. The expression of 84 genes known to be involved in the Wnt signalling pathway was tested by reverse transcription-polymerase chain reaction (RT-PCR) with a pathway-targeted array (Human Wnt RT(2) Profiler PCR Array, Superarray).

RESULTS

Using RT-PCR arrays, LEF1, PITX2 and secreted frizzled-related protein 1 (SFRP1) were upregulated more than twofold in KC compared with control epithelium. Only SFRP1 was significantly upregulated, approximately 25-fold compared with pooled controls (range 9.12-fold to 98.6-fold; P = 0.019). SFRP1 expression was associated with patient age and possibly the rate of progression of the keratoconus. Immunohistochemistry was used to assess SFRP1 protein distribution and confirm the SFRP1 microarray result (n = 3 KC and n = 2 control corneas). SFRP1 immunolablelling was seen in all KC corneas, mostly in the basal epithelium; however, control corneas showed minimal SFRP1 immunoreactivity.

CONCLUSION

SFRP1 is highly upregulated in the epithelium of these KC patients, suggesting a role in the pathogenesis and progression of keratoconus. Future investigations are required to establish if SFRP1 may be a potential marker of KC progression or if manipulation of its expression can be used to therapeutic effect in this disease.

Lens morphogenesis is dependent on Pax6-mediated inhibition of the canonical Wnt/beta-catenin signaling in the lens surface ectoderm

Lens formation in mouse is critically dependent on proper development of the retinal neuroectoderm that is located close beneath the head surface ectoderm. Signaling from the prospective retina triggers lens-specific gene expression in the surface-ectoderm. Supression of canonical Wnt/beta-catenin signaling in the surface ectoderm is one of the prerequisites for lens development because, as we show here, ectopic Wnt activation in the retina and lens abrogates lens formation. Wnt inhibiton is mediated by signals coming from the retina but its exact mechanism is unknown. We show that Pax6 directly controls expression of several Wnt inhibitors such as Sfrp1, Sfrp2, and Dkk1 in the presumptive lens. In accordance, absence of Pax6 function leads to aberrant canonical Wnt activity in the presumptive lens that subsequently impairs lens development. Thus Pax6 is required for down-regulation of canonical Wnt signaling in the presumptive lens ectoderm.

The regulation of Dkk1 expression during embryonic development

During embryogenesis, the Dkk1 mediated Wnt inhibition controls the spatiotemporal dynamics of cell fate determination, cell differentiation and cell death. Furthermore, the Dkk1 dose is critical for the normal Wnt homeostasis, as alteration of the Dkk1 activity is associated with various diseases. We investigated the regulation of Dkk1 expression during embryonic development. We identified nine conserved non-coding elements (CNEs), located 3' to the Dkk1 locus. Analyses of the regulatory potential revealed that four of these CNEs in combination drive reporter expression very similar to Dkk1 expression in several organs of transgenic embryos. We extended the knowledge of Dkk1 expression during hypophysis, external genitalia and kidney development, suggesting so far to unexplored functions of Dkk1 during the development of these organs. Characterization of the regulatory potential of four individual CNEs revealed that each of these promotes Dkk1 expression in brain and kidney. In combination, two enhancers are responsible for expression in the pituitary and the genital tubercle. Furthermore, individual CNEs mediates craniofacial, optic cup and limb specific Dkk1 regulation. Our study substantially improves the knowledge of Dkk1 regulation during embryonic development and thus might be of high relevance for therapeutic approaches.

Signaling "cross-talk" is integrated by transcription factors in the development of the anterior segment in the eye

Extracellular signaling "cross-talk" between tissues is an important requirement for development of many organs yet the underlying mechanisms generally remain poorly understood. The anterior segment of the eye, which is constructed from four embryonic lineages, provides a unique opportunity to genetically dissect developmental processes such as signaling "cross-talk" without fear of inducing lethality. In the current review, we summarize recent data showing that PITX2, a homeodomain transcription factor, integrates retinoic acid and canonical Wnt/beta-catenin signaling during anterior segment development. Because the requirements for retinoic acid signaling, canonical Wnt/beta-catenin signaling, and PITX2 are not unique to the eye, this newly identified pathway may have relevance elsewhere during development and in tissue homeostasis.

Gene expression profiles of lens regeneration and development in Xenopus laevis

Seven hundred and thirty-four unique genes were recovered from a cDNA library enriched for genes up-regulated during the process of lens regeneration in the frog Xenopus laevis. The sequences represent transcription factors, proteins involved in RNA synthesis/processing, components of prominent cell signaling pathways, genes involved in protein processing, transport, and degradation (e.g., the ubiquitin/proteasome pathway), matrix metalloproteases (MMPs), as well as many other proteins. The findings implicate specific signal transduction pathways in the process of lens regeneration, including the FGF, TGF-beta, MAPK, Retinoic acid, Wnt, and hedgehog signaling pathways, which are known to play important roles in eye/lens development and regeneration in various systems. In situ hybridization revealed that the majority of genes recovered are expressed during embryogenesis, including in eye tissues. Several novel genes specifically expressed in lenses were identified. The suite of genes was compared to those up-regulated in other regenerating tissues/organisms, and a small degree of overlap was detected.

Pax6 is essential for lens fiber cell differentiation

The developing ocular lens provides an excellent model system with which to study the intrinsic and extrinsic cues governing cell differentiation. Although the transcription factors Pax6 and Sox2 have been shown to be essential for lens induction, their later roles during lens fiber differentiation remain largely unknown. Using Cre/loxP mutagenesis, we somatically inactivated Pax6 and Sox2 in the developing mouse lens during differentiation of the secondary lens fibers and explored the regulatory interactions of these two intrinsic factors with the canonical Wnt pathway. Analysis of the Pax6-deficient lenses revealed a requirement for Pax6 in cell cycle exit and differentiation into lens fiber cells. In addition, Pax6 disruption led to apoptosis of lens epithelial cells. We show that Pax6 regulates the Wnt antagonist Sfrp2 in the lens, and that Sox2 expression is upregulated in the Pax6-deficient lenses. However, our study demonstrates that the failure of differentiation following loss of Pax6 is independent of beta-catenin signaling or Sox2 activity. This study reveals that Pax6 is pivotal for initiation of the lens fiber differentiation program in the mammalian eye.

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

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

Overexpression of Dickkopf 3 in hepatoblastomas and hepatocellular carcinomas

Dickkopf 3 (Dkk3) is a protein expressed at a very early stage of hepatogenesis. In this study, we examined whether Dkk3 was related to a premature or dedifferentiated nature in hepatoblastomas (HBLs) and hepatocellular carcinomas (HCCs). It was demonstrated that Dkk3 was overexpressed in HBLs and HCCs and that its expression was more frequent in the former than in the latter, being consistent with the fact that most HBLs show an embryonal or fetal hepatic histology, whereas there was no distinct relationship between Dkk3 expression and clinical data or histology. All of the HBLs expressed Dkk3, alpha-fetoprotein (AFP), or both proteins, suggesting that, similar to AFP, Dkk3 is another potentially useful biomarker detecting a wide range of HBLs. Furthermore, Dkk3 and AFP were expressed reciprocally in the tumors. These results suggest that Dkk3 may be related to the premature or dedifferentiated nature of HBLs and HCCs, whereas AFP may be related to a more differentiated nature. Thus, assessment of Dkk3 and AFP may be useful in the diagnosis of hepatic tumors.

Noncanonical Wnt signaling orchestrates early developmental events toward hematopoietic cell fate from human embryonic stem cells

During human development, signals that govern lineage specification versus expansion of cells committed to a cell fate are poorly understood. We demonstrate that activation of canonical Wnt signaling by Wnt3a promotes proliferation of human embryonic stem cells (hESCs)--precursors already committed to the hematopoietic lineage. In contrast, noncanonical Wnt signals, activated by Wnt11, control exit from the pluripotent state and entry toward mesoderm specification. Unique to embryoid body (EB) formation of hESCs, Wnt11 induces development and arrangement of cells expressing Brachyury that coexpress E-cadherin and Frizzled-7 (Fzd7). Knockdown of Fzd7 expression blocks Wnt11-dependent specification. Our study reveals an unappreciated role for noncanonical Wnt signaling in hESC specification that involves development of unique mesoderm precursors via morphogenic organization within human EBs.

CUX1/Wnt signaling regulates epithelial mesenchymal transition in EBV infected epithelial cells. Exp Cell Res. 2009;315:1819-1831. [PMID: 19361498 DOI: 10.1016/j.yexcr.2009.04.001]

Idiopathic pulmonary fibrosis (IPF) is a refractory and lethal interstitial lung disease characterized by alveolar epithelial cells apoptosis, fibroblast proliferation and extra-cellular matrix protein deposition. EBV, localised to alveolar epithelial cells of pulmonary fibrosis patients is associated with a poor prognosis. A strategy based on microarray-differential gene expression analysis to identify molecular drivers of EBV-associated lung fibrosis was utilized. Alveolar epithelial cells were infected with EBV to identify genes whose expression was altered following TGFbeta1-mediated lytic phase. EBV lytic reactivation by TGFbeta1 drives a selective alteration in CUX1 variant (a) (NCBI accession number NM_181552) expression, inducing activation of non-canonical Wnt pathway mediators, implicating it in Epithelial Mesenchymal Transition (EMT), the molecular event underpinning scar production in tissue fibrosis. The role of EBV in EMT can be attenuated by antiviral strategies and inhibition of Wnt signaling by using All-Trans Retinoic Acids (ATRA). Activation of non-canonical Wnt signaling pathway by EBV in epithelial cells suggests a novel mechanism of EMT via CUX1 signaling. These data present a framework for further description of the link between infectious agents and fibrosis, a significant disease burden.

Wnt signaling is required for organization of the lens fiber cell cytoskeleton and development of lens three-dimensional architecture

How an organ develops its characteristic shape is a major issue. This is particularly critical for the eye lens as its function depends on having appropriately ordered three-dimensional cellular architecture. Recent in vitro studies indicate that Wnt signaling plays key roles in regulating morphological events in FGF-induced fiber cell differentiation in the mammalian lens. To further investigate this the Wnt signaling antagonist, secreted frizzled-related protein 2 (Sfrp2), was overexpressed in lens fiber cells of transgenic mice. In these mice fiber cell elongation was attenuated and individual fibers exhibited irregular shapes and consequently did not align or pack regularly; microtubules, microfilaments and intermediate filaments were clearly disordered in these fibers. Furthermore, a striking feature of transgenic lenses was that fibers did not develop the convex curvature typically seen in normal lenses. This appears to be related to a lack of protrusive processes that are required for directed migratory activity at their apical and basal tips as well as for the formation of interlocking processes along their lateral margins. Components of the Wnt/Planar Cell Polarity (PCP) pathway were downregulated or inhibited. Taken together this supports a role for Wnt/PCP signaling in orchestrating the complex organization and dynamics of the fiber cell cytoskeleton.

Deciphering the function of canonical Wnt signals in development and disease: conditional loss- and gain-of-function mutations of beta-catenin in mice

Wnt signaling is one of a handful of powerful signaling pathways that play crucial roles in the animal life by controlling the genetic programs of embryonic development and adult homeostasis. When disrupted, these signaling pathways cause developmental defects, or diseases, among them cancer. The gateway of the canonical Wnt pathway, which contains >100 genes, is an essential molecule called beta-catenin (Armadillo in Drosophila). Conditional loss- and gain-of-function mutations of beta-catenin in mice provided powerful tools for the functional analysis of canonical Wnt signaling in many tissues and organs. Such studies revealed roles of Wnt signaling that were previously not accessible to genetic analysis due to the early embryonic lethality of conventional beta-catenin knockout mice, as well as the redundancy of Wnt ligands, receptors, and transcription factors. Analysis of conditional beta-catenin loss- and gain-of-function mutant mice demonstrated that canonical Wnt signals control progenitor cell expansion and lineage decisions both in the early embryo and in many organs. Canonical Wnt signaling also plays important roles in the maintenance of various embryonic or adult stem cells, and as recent findings demonstrated, in cancer stem cell types. This has opened new opportunities to model numerous human diseases, which have been associated with deregulated Wnt signaling. Our review summarizes what has been learned from genetic studies of the Wnt pathway by the analysis of conditional beta-catenin loss- and gain-of-function mice.

Expression and regulation of Dickkopf2 during periimplantation in mice

Successful implantation depends on active dialogue between the maternal endometrium and the implanting blastocysts that is well controlled by groups of regulators at the molecular level. Dickkopf2 (Dkk2) is a member of Dickkopf family normally acting as an antagonist of canonical Wnt/beta-catenin signaling, which has been proven to participate in tumorigenesis and early embryo development. In order to explore the potential function of Dkk2 in embryo implantation, the present study investigated the uterine expression and regulation profiles of Dkk2 during periimplantation in mice. Using reverse transcription-polymerase chain reaction, immunohistochemistry and Western blotting, we showed that the mRNA and protein levels of Dkk2 began to increase in the glandular epithelium on day 4, continued to increase on day 5 and then decreased from day 6 of pregnancy. Moreover, on days 5-8 of pregnancy, Dkk2 was increasingly expressed in the deciduum of the uterus, especially around the implanting embryos. In addition, upregulation of Dkk2 was also observed in uteri treated with estrogen (estradiol-17beta) as well as in oil-induced artificial decidualization, indicating that the expression of Dkk2 could be induced by both steroid hormone (estrogen) and the process of decidualization. Furthermore, in the postimplantation uterus, the Dkk2 protein showed an inversed expression with active beta-catenin from day 6 onward, supporting the notion that Dkk2 plays an inhibitory role against canonical Wnt signaling in the context of the decidualizing stroma. Collectively, our data suggests that Dkk2 expression is associated with uterine receptivity changes as well as the process of decidualization and that it might play important roles through inhibition of canonical Wnt signaling in the periimplantation uterus.

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

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

Dkk3 is required for TGF-β signaling during Xenopus mesoderm induction

The Dickkopf (Dkk) family is composed of four main members (Dkk1-4), which typically regulate Wnt/beta-catenin signaling. An exception is Dkk3, which does not affect Wnt/beta-catenin signaling and whose function is poorly characterized. Here, we describe the Xenopus dkk3 homolog and characterize its expression and function during embryogenesis. Dkk3 is maternally expressed and zygotically in the cement gland, head mesenchyme, and heart. We show that depletion of Dkk3 in Xenopus embryos by Morpholino antisense oligonucleotides induces axial defects as a result of Spemann organizer and mesoderm inhibition. Dkk3 depletion leads to down-regulation of Activin/Nodal signaling by reducing levels of Smad4 protein. Dkk3 overexpression can rescue phenotypic effects resulting from overexpression of the Smad4 ubiquitin ligase Ectodermin. Furthermore, depletion of Dkk3 up-regulates FGF signaling, while Dkk3 overexpression reduces it. These results indicate that Dkk3 modulates FGF and Activin/Nodal signaling to regulate mesoderm induction during early Xenopus development.

Dkk3-Cre BAC transgenic mouse line: a tool for highly efficient gene deletion in retinal progenitor cells

To establish the genetic tools for conditional gene deletion in mouse retinal progenitors, we generated a Dkk3-Cre transgenic mouse line using bacterial artificial chromosome (BAC) transgenesis. Cre recombination efficiency in vivo was assayed by crossing this transgenic line, termed BAC-Dkk3-Cre, with the CAG-CAT-Z reporter line. This BAC-Dkk3-Cre line showed Cre recombinase activity in most retinal progenitors. Cre activity was detectable from embryonic day 10.5 (E10.5) and generally restricted to the retina during embryogenesis. To verify that BAC-Dkk3-Cre mice successfully circumvented lethality, we generated Otx2flox/flox/BAC-Dkk3-Cre+ mice as Otx2 conditional knockout mice. The Otx2flox/flox/BAC-Dkk3-Cre+ mice were viable, and their retina showed loss of mature cell-type markers of photoreceptor cells, bipolar cells, and horizontal cells, in contrast, amacrine-like cells noticeably increased. Thus, the BAC-Dkk3-Cre transgenic mouse line provides a powerful tool for generating conditional knockout mouse lines for studying loss of gene functions in the developing retina.

Abnormal lens morphogenesis and ectopic lens formation in the absence of β-catenin function

beta-Catenin plays a key role in cadherin-mediated cell adhesion as well as in canonical Wnt signaling. To study the role of beta-catenin during eye development, we used conditional Cre/loxP system in mouse to inactivate beta-catenin in developing lens and retina. Inactivation of beta-catenin does not suppress lens fate, but instead results in abnormal morphogenesis of the lens. Using BAT-gal reporter mice, we show that beta-catenin-mediated Wnt signaling is notably absent from lens and neuroretina throughout eye development. The observed defect is therefore likely due to the cytoskeletal role of beta-catenin, and is accompanied by impaired epithelial cell adhesion. In contrast, inactivation of beta-catenin in the nasal ectoderm, an area with active Wnt signaling, results in formation of crystallin-positive ectopic lentoid bodies. These data suggest that, outside of the normal lens, beta-catenin functions as a coactivator of canonical Wnt signaling to suppress lens fate.