AP-2alpha transcription factor is required for early morphogenesis of the lens vesicle

Clinical and Genetic Correlation in Neurocristopathies: Bridging a Precision Medicine Gap

Neurocristopathies (NCPs) encompass a spectrum of disorders arising from issues during the formation and migration of neural crest cells (NCCs). NCCs undergo epithelial-mesenchymal transition (EMT) and upon key developmental gene deregulation, fetuses and neonates are prone to exhibit diverse manifestations depending on the affected area. These conditions are generally rare and often have a genetic basis, with many following Mendelian inheritance patterns, thus making them perfect candidates for precision medicine. Examples include cranial NCPs, like Goldenhar syndrome and Axenfeld-Rieger syndrome; cardiac-vagal NCPs, such as DiGeorge syndrome; truncal NCPs, like congenital central hypoventilation syndrome and Waardenburg syndrome; and enteric NCPs, such as Hirschsprung disease. Additionally, NCCs' migratory and differentiating nature makes their derivatives prone to tumors, with various cancer types categorized based on their NCC origin. Representative examples include schwannomas and pheochromocytomas. This review summarizes current knowledge of diseases arising from defects in NCCs' specification and highlights the potential of precision medicine to remedy a clinical phenotype by targeting the genotype, particularly important given that those affected are primarily infants and young children.

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.

Corneal epithelial development and homeostasis

The corneal epithelium (CE), the most anterior cellular structure of the eye, is a self-renewing stratified squamous tissue that protects the rest of the eye from external elements. Each cell in this exquisite three-dimensional structure needs to have proper polarity and positional awareness for the CE to serve as a transparent, refractive, and protective tissue. Recent studies have begun to elucidate the molecular and cellular events involved in the embryonic development, post-natal maturation, and homeostasis of the CE, and how they are regulated by a well-coordinated network of transcription factors. This review summarizes the status of related knowledge and aims to provide insight into the pathophysiology of disorders caused by disruption of CE development, and/or homeostasis.

Generation of Lens Progenitor Cells and Lentoid Bodies from Pluripotent Stem Cells: Novel Tools for Human Lens Development and Ocular Disease Etiology

In vitro differentiation of human pluripotent stem cells (hPSCs) into specialized tissues and organs represents a powerful approach to gain insight into those cellular and molecular mechanisms regulating human development. Although normal embryonic eye development is a complex process, generation of ocular organoids and specific ocular tissues from pluripotent stem cells has provided invaluable insights into the formation of lineage-committed progenitor cell populations, signal transduction pathways, and self-organization principles. This review provides a comprehensive summary of recent advances in generation of adenohypophyseal, olfactory, and lens placodes, lens progenitor cells and three-dimensional (3D) primitive lenses, "lentoid bodies", and "micro-lenses". These cells are produced alone or "community-grown" with other ocular tissues. Lentoid bodies/micro-lenses generated from human patients carrying mutations in crystallin genes demonstrate proof-of-principle that these cells are suitable for mechanistic studies of cataractogenesis. Taken together, current and emerging advanced in vitro differentiation methods pave the road to understand molecular mechanisms of cataract formation caused by the entire spectrum of mutations in DNA-binding regulatory genes, such as PAX6, SOX2, FOXE3, MAF, PITX3, and HSF4, individual crystallins, and other genes such as BFSP1, BFSP2, EPHA2, GJA3, GJA8, LIM2, MIP, and TDRD7 represented in human cataract patients.

A New Case and Comprehensive Review of the Ophthalmic Manifestations of 172 Individuals With Branchio-Oculo-Facial Syndrome

PURPOSE

To review the literature on branchio-oculo-facial syndrome and describe a new case.

METHODS

A girl presented with a de novo pathogenic mutation in the TFAP2A gene consistent with branchiooculo-facial syndrome. A systematic review was also performed to characterize the eye manifestations associated with the syndrome.

RESULTS

A total of 172 total patients were identified from the literature. Among these, 102 patients received molecular confirmation. The most common pathogenic variants reported were p.R255G, p.A256V, p.R254W, and p.G251E. Common eye abnormalities associated with the syndrome in total combined cases (represents individuals with a clinical diagnosis only of branchiooculo-facial syndrome plus those who additionally had molecular confirmation of the syndrome from genetic testing) were nasolacrimal duct stenosis (n = 98, 57%), coloboma (n = 76, 46%), anophthalmia/microphthalmia (n = 64, 37%), and cataracts (n = 27, 16%).

CONCLUSIONS

This analysis provides a comprehensive review of genetic variants and ophthalmic findings to characterize the most common eye manifestations associated with branchio-oculo-facial syndrome. The report provides incentive to further investigate TFAP2A variants and identify genotype-phenotype correlations. [J Pediatr Ophthalmol Strabismus. 20XX;XX(X):XX-XX.].

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.

Deletion of transcription factor AP-2β from the developing murine trabecular meshwork region leads to progressive glaucomatous changes

Glaucoma is one of the leading causes of irreversible blindness and can result from abnormalities in anterior segment structures required for aqueous humor outflow, including the trabecular meshwork (TM) and Schlemm's canal (SC). Transcription factors such as AP-2β play critical roles in anterior segment development. Here, we show that the Mgp-Cre knock-in (Mgp-Cre.KI) mouse can be used to target the embryonic periocular mesenchyme giving rise to the TM and SC. Fate mapping of male and female mice indicates that AP-2β loss causes a decrease in iridocorneal angle cells derived from Mgp-Cre.KI-expressing populations compared to controls. Moreover, histological analyses revealed peripheral iridocorneal adhesions in AP-2β mutants that were accompanied by a decrease in expression of TM and SC markers, as observed using immunohistochemistry. In addition, rebound tonometry showed significantly higher intraocular pressure (IOP) that was correlated with a progressive significant loss of retinal ganglion cells, reduced retinal thickness, and reduced retinal function, as measured using an electroretinogram, in AP-2β mutants compared with controls, reflecting pathology described in late-stage glaucoma patients. Importantly, elevated IOP in AP-2β mutants was significantly reduced by treatment with latanoprost, a prostaglandin analog that increases unconventional outflow. These findings demonstrate that AP-2β is critical for TM and SC development, and that these mutant mice can serve as a model for understanding and treating progressive human primary angle-closure glaucoma.

Conditional Deletion of AP-2β in the Periocular Mesenchyme of Mice Alters Corneal Epithelial Cell Fate and Stratification

The cornea is an anterior eye structure specialized for vision. The corneal endothelium and stroma are derived from the periocular mesenchyme (POM), which originates from neural crest cells (NCCs), while the stratified corneal epithelium develops from the surface ectoderm. Activating protein-2β (AP-2β) is highly expressed in the POM and important for anterior segment development. Using a mouse model in which AP-2β is conditionally deleted in the NCCs (AP-2β NCC KO), we investigated resulting corneal epithelial abnormalities. Through PAS and IHC staining, we observed structural and phenotypic changes to the epithelium associated with AP-2β deletion. In addition to failure of the mutant epithelium to stratify, we also observed that Keratin-12, a marker of the differentiated epithelium, was absent, and Keratin-15, a limbal and conjunctival marker, was expanded across the central epithelium. Transcription factors PAX6 and P63 were not observed to be differentially expressed between WT and mutant. However, growth factor BMP4 was suppressed in the mutant epithelium. Given the non-NCC origin of the epithelium, we hypothesize that the abnormalities in the AP-2β NCC KO mouse result from changes to regulatory signaling from the POM-derived stroma. Our findings suggest that stromal pathways such as Wnt/β-Catenin signaling may regulate BMP4 expression, which influences cell fate and stratification.

Brahma-related gene 1 has time-specific roles during brain and eye development

During development, gene expression is tightly controlled to facilitate the generation of the diverse cell types that form the central nervous system. Brahma-related gene 1 (Brg1, also known as Smarca4) is the catalytic subunit of the SWItch/sucrose nonfermentable (SWI/SNF) chromatin remodeling complex that regulates transcription. We investigated the role of Brg1 between embryonic day 6.5 (E6.5) and E14.5 in Sox2-positive neural stem cells (NSCs). Being without major consequences at E6.5 and E14.5, loss of Brg1 between E7.5 and E12.5 resulted in the formation of rosette-like structures in the subventricular zone, as well as morphological alterations and enlargement of neural retina (NR). Additionally, Brg1-deficient cells showed decreased survival in vitro and in vivo. Furthermore, we uncovered distinct changes in gene expression upon Brg1 loss, pointing towards impaired neuron functions, especially those involving synaptic communication and altered composition of the extracellular matrix. Comparison with mice deficient for integrase interactor 1 (Ini1, also known as Smarcb1) revealed that the enlarged NR was Brg1 specific and was not caused by a general dysfunction of the SWI/SNF complex. These results suggest a crucial role for Brg1 in NSCs during brain and eye development.

AP-2β is required for formation of the murine trabecular meshwork and Schlemm's canal

Previously, we have shown that Tfap2b, the gene encoding transcription factor AP-2β, is needed for normal mouse eye development. Specifically, targeted loss of Tfap2b in neural crest cells (NCCs)¹ and their derivatives, particularly the periocular mesenchyme (POM), resulted in anterior segment defects affecting the cornea and angle tissue. These defects were further associated with an increase in intraocular pressure (IOP). The present study investigates the underlying changes in embryonic and postnatal POM cell development and differentiation caused by loss of AP-2β in the NCCs, particularly in the structures that control aqueous outflow, using Wnt1Cre^+/-; Tfap2b^-/lox; tdTomato^lox/+ mice (AP-2β neural crest cell knockout or AP-2β NCC KO). Toluidine blue-stained sections and ultrathin sections stained with uranyl acetate and lead citrate were used to assess morphology and ultrastructure, respectively. Immunohistochemistry of KO and control eyes was performed at embryonic day (E) 15.5, E18.5, postnatal day (P) 1, P7 and P14 using phospho-histone H3 (PH3), α-smooth muscle actin (α-SMA), myocilin and endomucin antibodies, as well as a TUNEL assay. Conditional deletion of AP-2β in the NCC-derived POM resulted in defects that appeared during both embryogenesis and postnatal stages. Fate mapping of the knockout cells in the mutants revealed that the POM migrated appropriately into the eye during embryogenesis. However, during postnatal stages a significant reduction in POM proliferation in the angle region was observed in the mutants compared to controls. This was accompanied by a lack of expression of appropriate trabecular meshwork and Schlemm's canal markers. This is the first study to show that AP-2β is required for development and differentiation of the trabecular meshwork and Schlemm's canal. Together, these defects likely contributed to the elevated intraocular pressure (IOP) previously reported in the AP-2β NCC KO mice.

Protein kinase D up-regulates transcription of VEGF receptor-2 in endothelial cells by suppressing nuclear localization of the transcription factor AP2β

Vascular endothelial growth factor A (VEGF) signals primarily through its cognate receptor VEGF receptor-2 (VEGFR-2) to control vasculogenesis and angiogenesis, key physiological processes in cardiovascular disease and cancer. In human umbilical vein endothelial cells (HUVECs), knockdown of protein kinase D-1 (PKD1) or PKD2 down-regulates VEGFR-2 expression and inhibits VEGF-induced cell proliferation and migration. However, how PKD regulates VEGF signaling is unclear. Previous bioinformatics analyses have identified binding sites for the transcription factor activating enhancer-binding protein 2 (AP2) in the VEGFR-2 promoter. Using ChIP analyses, here we found that PKD knockdown in HUVECs increases binding of AP2β to the VEGFR-2 promoter. Luciferase reporter assays with serial deletions of AP2-binding sites within the VEGFR-2 promoter revealed that its transcriptional activity negatively correlates with the number of these sites. Next we demonstrated that AP2β up-regulation decreases VEGFR-2 expression and that loss of AP2β enhances VEGFR-2 expression in HUVECs. In vivo experiments confirmed increased VEGFR-2 immunostaining in the spinal cord of AP2β knockout mouse embryos. Mechanistically, we observed that PKD phosphorylates AP2β at Ser258 and Ser277 and suppresses its nuclear accumulation. Inhibition of PKD activity with a pan-PKD inhibitor increased AP2β nuclear localization, and overexpression of both WT and constitutively active PKD1 or PKD2 reduced AP2β nuclear localization through a Ser258- and Ser277-dependent mechanism. Furthermore, substitution of Ser277 in AP2β increased its binding to the VEGFR-2 promoter. Our findings uncover evidence of a molecular pathway that regulates VEGFR-2 expression, insights that may shed light on the etiology of diseases associated with aberrant VEGF/VEGFR signaling.

SCF/SCFR signaling plays an important role in the early morphogenesis and neurogenesis of human embryonic neural retina

The stem cell factor receptor (SCFR) has been demonstrated to be expressed in the neural retina of mice, rat and human for decades. Previous reports indicated that the SCFR correlates with glia differentiation of late retinal progenitor cells (RPCs), retinal vasculogenesis and homeostasis of the blood-retinal barrier. However, the role of SCF/SCFR signaling in the growth and development of the neural retina (NR), especially in the early embryonic stage, remains poorly understood. Here, we show that SCF/SCFR signaling orchestrates invagination of the human embryonic stem cell (hESC)-derived NR via regulation of cell cycle progression, cytoskeleton dynamic and apical constriction of RPCs in the ciliary marginal zone (CMZ). Furthermore, activation of SCF/SCFR signaling promotes neurogenesis in the central-most NR via acceleration of the migration of immature ganglion cells and repressing apoptosis. Our study reveals an unreported role for SCF/SCFR signaling in controlling ciliary marginal cellular behaviors during early morphogenesis and neurogenesis of the human embryonic NR, providing a new potential therapeutic target for human congenital eye diseases such as anophthalmia, microphthalmia and congenital high myopia.

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.

Conditional Deletion of AP-2α and AP-2β in the Developing Murine Retina Leads to Altered Amacrine Cell Mosaics and Disrupted Visual Function

Purpose

The combined action of the activating protein-2 (AP-2) transcription factors, AP-2α and AP-2β, is important in early retinal development, specifically in the formation of horizontal cells. However, in previous studies, it was not possible to analyze postnatal development and function of additional retinal subtypes.

Methods

We used a double conditional deletion of AP-2α and AP-2β from the retina to further examine the combinatory role of these genes in retinal cell patterning and function in postnatal adult mice as measured by Voronoi domain area and nearest-neighbor distance spatial analyses and ERGs, respectively.

Results

Conditional deletion of both AP-2α and AP-2β from the retina resulted in a variety of abnormalities, including the absence of horizontal cells, defects in the photoreceptor ribbons in which synapses failed to form, along with evidence of aberrant amacrine cell arrangement. Although no significant changes in amacrine cell population numbers were observed in the double mutants, significant irregularities in the mosaic patterning of amacrine cells was observed as demonstrated by both Voronoi domain areas and nearest-neighbor distances analyses. These changes were further accompanied by an alteration in the retinal response to light as recorded by ERGs. In particular, in the double-mutant mice lacking AP-2α and AP-2β, the b-wave amplitude, representative of interneuron signal processing, was significantly reduced compared with control littermates.

Conclusions

Together these findings demonstrate the requirement for both AP-2α and AP-2β in proper amacrine mosaic patterning and a normal functional light response in the retina.

Lens-specific deletion of the Msx2 gene increased apoptosis by enhancing the caspase-3/caspase-8 signaling pathway

Objective To investigate the influence of Msx2 conditional gene knockout during lens development in mice. Methods Lens-specific Msx2 knockout mice were generated using the Cre-loxP system. The eyes of Msx2 conditional knockout ( Msx2CKO) and wild-type ( Msx2WT) mice were examined during embryonic and early postnatal periods using histological, immunofluorescence, in situ hybridization, cell proliferation, apoptosis, and mRNA microarray analyses. Results Msx2CKO mice exhibited small lens formation and microphthalmia after birth, while Msx2CKO embryos exhibited a persistent lens stalk, small lens formation, and microphthalmia. Conditional deletion of Msx2 also led to an increased apoptosis rate, a significant reduction in FoxE3 expression, and an upregulation of Prox1 expression in the lens vesicle during the early embryonic period. Microarray comparison of Msx2CKO and Msx2WT lens transcriptomes identified a large number of differentially expressed genes. Real-time PCR showed that Casp8 and Casp3 expression was upregulated in Msx2CKO mice at post-natal day 1. Conclusion The activation of apoptosis through the caspase-8/caspase-3 signaling pathway, together with the downregulation of FoxE3 expression, appeared to account for the smaller lens formation in Msx2CKO mice.

The cellular bases of choroid fissure formation and closure

Defects in choroid fissure (CF) formation and closure lead to coloboma, a major cause of childhood blindness. Despite genetic advances, the cellular defects underlying coloboma remain poorly elucidated due to our limited understanding of normal CF morphogenesis. We address this deficit by conducting high-resolution spatio-temporal analyses of CF formation and closure in the chick, mouse and fish. We show that a small ventral midline invagination initiates CF formation in the medial-proximal optic cup, subsequently extending it dorsally toward the lens, and proximally into the optic stalk. Unlike previously supposed, the optic disc does not form solely as a result of this invagination. Morphogenetic events that alter the shape of the proximal optic cup also direct clusters of outer layer and optic stalk cells to form dorsal optic disc. A cross-species comparison suggests that CF closure can be accomplished by breaking down basement membranes (BM) along the CF margins, and by establishing BM continuity along the dorsal and ventral surfaces of the CF. CF closure is subsequently accomplished via two distinct mechanisms: tissue fusion or the intercalation of various tissues into the inter-CF space. We identify several novel cell behaviors that underlie CF fusion, many of which involve remodeling of the retinal epithelium. In addition to BM disruption, these include NCAD downregulation along the SOX2⁺ retinal CF margin, and the protrusion or movement of partially polarized retinal cells into the inter-CF space to mediate fusion. Proximally, the inter-CF space does not fuse or narrow and is instead loosely packed with migrating SOX2⁺/PAX2⁺/Vimentin⁺ astrocytes until it is closed by the outgoing optic nerve. Taken together, our results highlight distinct proximal-distal differences in CF morphogenesis and closure and establish detailed cellular models that can be utilized for understanding the genetic bases of coloboma.

Relationship between neural crest cell specification and rare ocular diseases

Development of the eye is closely associated with neural crest cell migration and specification. Eye development is extremely complex, as it requires the working of a combination of local factors, receptors, inductors, and signaling interactions between tissues such as the optic cup and periocular mesenchyme (POM). The POM is comprised of neural crest-derived mesenchymal progenitor cells that give rise to numerous important ocular structures including those tissues that form the optic cup and anterior segment of the eye. A number of genes are involved in the migration and specification of the POM such as PITX2, PITX3, FOXC1, FOXE3, PAX6, LMX1B, GPR48, TFAP2A, and TFAP2B. In this review, we will discuss the relevance of these genes in the development of the POM and how mutations and defects result in rare ocular diseases.

AP-2ε Expression in Developing Retina: Contributing to the Molecular Diversity of Amacrine Cells

AP-2 transcription factors play important roles in the regulation of gene expression during development. Four of the five members of the AP-2 family (AP-2α, AP-2β, AP-2γ and AP-2δ) have previously been shown to be expressed in developing retina. Mouse knockouts have revealed roles for AP-2α, AP-2β and AP-2δ in retinal cell specification and function. Here, we show that the fifth member of the AP-2 family, AP-2ε, is also expressed in amacrine cells in developing mammalian and chicken retina. Our data indicate that there are considerably fewer AP-2ε-positive cells in the developing mouse retina compared to AP-2α, AP-2β and AP-2γ-positive cells, suggesting a specialized role for AP-2ε in a subset of amacrine cells. AP-2ε, which is restricted to the GABAergic amacrine lineage, is most commonly co-expressed with AP-2α and AP-2β, especially at early stages of retinal development. Co-expression of AP-2ε and AP-2γ increases with differentiation. Analysis of previously published Drop-seq data from single retinal cells supports co-expression of multiple AP-2s in the same cell. Since AP-2s bind to their target sequences as either homodimers or heterodimers, our work suggests spatially- and temporally-coordinated roles for combinations of AP-2 transcription factors in amacrine cells during retinal development.

Generating retinoic acid gradients by local degradation during craniofacial development: One cell's cue is another cell's poison

Retinoic acid (RA) is a vital morphogen for early patterning and organogenesis in the developing embryo. RA is a diffusible, lipophilic molecule that signals via nuclear RA receptor heterodimeric units that regulate gene expression by interacting with RA response elements in promoters of a significant number of genes. For precise RA signaling, a robust gradient of the morphogen is required. The developing embryo contains regions that produce RA, and specific intracellular concentrations of RA are created through local degradation mediated by Cyp26 enzymes. In order to elucidate the mechanisms by which RA executes precise developmental programs, the kinetics of RA metabolism must be clearly understood. Recent advances in techniques for endogenous RA detection and quantification have paved the way for mechanistic studies to shed light on downstream gene expression regulation coordinated by RA. It is increasingly coming to light that RA signaling operates not only at precise concentrations but also employs mechanisms of degradation and feedback inhibition to self-regulate its levels. A global gradient of RA throughout the embryo is often found concurrently with several local gradients, created by juxtaposed domains of RA synthesis and degradation. The existence of such local gradients has been found especially critical for the proper development of craniofacial structures that arise from the neural crest and the cranial placode populations. In this review, we summarize the current understanding of how local gradients of RA are established in the embryo and their impact on craniofacial development.

Generation of a new mouse model of glaucoma characterized by reduced expression of the AP-2β and AP-2δ proteins

We generated 6 transgenic lines with insertion of an expression plasmid for the R883/M xanthine dehydrogenase (XDH) mutant protein. Approximately 20% of the animals deriving from one of the transgenic lines show ocular abnormalities and an increase in intra-ocular pressure which are consistent with glaucoma. The observed pathologic phenotype is not due to expression of the transgene, but rather the consequence of the transgene insertion site, which has been defined by genome sequencing. The insertion site maps to chromosome 1qA3 in close proximity to the loci encoding AP-2β and AP-2δ, two proteins expressed in the eye. The insertion leads to a reduction in AP-2β and AP-2δ levels. Down-regulation of AP-2β expression is likely to be responsible for the pathologic phenotype, as conditional deletion of the Tfap2b gene in the neural crest has recently been shown to cause defective development of the eye anterior segment and early-onset glaucoma. In these conditional knock-out and our transgenic mice, the morphological/histological features of the glaucomatous pathology are surprisingly similar. Our transgenic mouse represents a model of angle-closure glaucoma and a useful tool for the study of the pathogenesis and the development of innovative therapeutic strategies.

Distinct Activities of Tfap2A and Tfap2B in the Specification of GABAergic Interneurons in the Developing Cerebellum

GABAergic inhibitory neurons in the cerebellum are subdivided into Purkinje cells and distinct subtypes of interneurons from the same pool of progenitors, but the determinants of this diversification process are not well defined. To explore the transcriptional regulation of the development of cerebellar inhibitory neurons, we examined the role of Tfap2A and Tfap2B in the specification of GABAergic neuronal subtypes in mice. We show that Tfap2A and Tfap2B are expressed in inhibitory precursors during embryonic development and that their expression persists into adulthood. The onset of their expression follows Ptf1a and Olig2, key determinants of GABAergic neuronal fate in the cerebellum; and, their expression precedes Pax2, an interneuron-specific factor. Tfap2A is expressed by all GABAergic neurons, whereas Tfap2B is selectively expressed by interneurons. Genetic manipulation via in utero electroporation (IUE) reveals that Tfap2B is necessary for interneuron specification and is capable of suppressing the generation of excitatory cells. Tfap2A, but not Tfap2B, is capable of inducing the generation of interneurons when misexpressed in the ventricular neuroepithelium. Together, our results demonstrate that the differential expression of Tfap2A and Tfap2B defines subtypes of GABAergic neurons and plays specific, but complementary roles in the specification of interneurons in the developing cerebellum.

Conditional deletion of AP-2β in mouse cranial neural crest results in anterior segment dysgenesis and early-onset glaucoma

Anterior segment dysgenesis (ASD) encompasses a group of developmental disorders in which a closed angle phenotype in the anterior chamber of the eye can occur and 50% of patients develop glaucoma. Many ASDs are thought to involve an inappropriate patterning and migration of the periocular mesenchyme (POM), which is derived from cranial neural crest cells (NCCs) and mesoderm. Although, the mechanism of this disruption is not well understood, a number of transcriptional regulatory molecules have previously been implicated in ASDs. Here, we investigate the function of the transcription factor AP-2β, encoded by Tfap2b, which is expressed in NCCs and their derivatives. Wnt1-Cre-mediated conditional deletion of Tfap2b in NCCs resulted in post-natal ocular defects typified by opacity. Histological data revealed that the conditional AP-2β NCC knockout (KO) mutants exhibited dysgenesis of multiple structures in the anterior segment of the eye including defects in the corneal endothelium, corneal stroma, ciliary body and disruption in the iridocorneal angle with adherence of the iris to the cornea. We further show that this phenotype leads to a significant increase in intraocular pressure and a subsequent loss of retinal ganglion cells and optic nerve degeneration, features indicative of glaucoma. Overall, our findings demonstrate that AP-2β is required in the POM for normal development of the anterior segment of the eye and that the AP-2β NCC KO mice might serve as a new and exciting model of ASD and glaucoma that is fully penetrant and with early post-natal onset.

Summary: Tissue-specific deletion of transcription factor AP-2β in the neural-crest-derived periocular mesenchyme generates a novel model of anterior segment dysgenesis and early onset glaucoma in mice.

The molecular mechanisms underlying lens fiber elongation

Lens fiber cells are highly elongated cells with complex membrane morphologies that are critical for the transparency of the ocular lens. Investigations into the molecular mechanisms underlying lens fiber cell elongation were first reported in the 1960s, however, our understanding of the process is still poor nearly 50 years later. This review summarizes what is currently hypothesized about the regulation of lens fiber cell elongation along with the available experimental evidence, and how this information relates to what is known about the regulation of cell shape/elongation in other cell types, particularly neurons.

AP-2β Is a Downstream Effector of PITX2 Required to Specify Endothelium and Establish Angiogenic Privilege During Corneal Development

PURPOSE

The homeodomain transcription factor, PITX2, is at the apex of a genetic pathway required for corneal development, but the critical effector genes regulated by the PITX2 remain unknown. The purpose of this study was to discover and validate PITX2-dependent mechanisms required for specifying cell lineages and establishing angiogenic privilege within the developing cornea.

METHODS

Microarrays were used to compare gene expression in corneas isolated from temporal Pitx2 knockout embryos and control littermates. Quantitative RT-PCR and immunohistochemistry was used to further validate Tfap2b expression differences in Pitx2 knockout versus control corneas. In situ hybridization and protein immunohistochemistry were used to assay eyes of a Tfap2b allelic series of embryos to identify differentiated cellular lineages in the cornea, blood vessel endothelium, or lymphatic vessel endothelium.

RESULTS

We show that PITX2 is required for the expression of Tfap2b, encoding the AP-2β transcription factor, in the neural crest during corneal development. Markers of differentiated corneal epithelium and stroma are expressed in the absence of AP-2β. In contrast, markers of differentiated corneal endothelium are not expressed in the absence of AP-2β. Endomucin+ blood vessels are present throughout the developing corneal stroma in the absence of AP-2β, whereas LYVE1+ lymphatic vessels are not found.

CONCLUSIONS

The AP-2β transcription factor is an important effector of PITX2 function during corneal development, required for differentiation of corneal endothelium and establishment of angiogenic privilege. Unlike PITX2, AP-2β is not required for the early expression of available lineage specific markers for the corneal epithelium and stroma during embryogenesis, nor establishment of lymphangiogenic privilege. Therefore, additional PITX2-dependent factors likely regulate these latter processes during embryonic development. These results extend our understanding of the genetic mechanisms regulating cornea development.

Edges of human embryonic stem cell colonies display distinct mechanical properties and differentiation potential

In order to understand the mechanisms that guide cell fate decisions during early human development, we closely examined the differentiation process in adherent colonies of human embryonic stem cells (hESCs). Live imaging of the differentiation process reveals that cells on the outer edge of the undifferentiated colony begin to differentiate first and remain on the perimeter of the colony to eventually form a band of differentiation. Strikingly, this band is of constant width in all colonies, independent of their size. Cells at the edge of undifferentiated colonies show distinct actin organization, greater myosin activity and stronger traction forces compared to cells in the interior of the colony. Increasing the number of cells at the edge of colonies by plating small colonies can increase differentiation efficiency. Our results suggest that human developmental decisions are influenced by cellular environments and can be dictated by colony geometry of hESCs.

Tfap2a and 2b act downstream of Ptf1a to promote amacrine cell differentiation during retinogenesis

Retinogenesis is a precisely controlled developmental process during which different types of neurons and glial cells are generated under the influence of intrinsic and extrinsic factors. Three transcription factors, Foxn4, RORβ1 and their downstream effector Ptf1a, have been shown to be indispensable intrinsic regulators for the differentiation of amacrine and horizontal cells. At present, however, it is unclear how Ptf1a specifies these two cell fates from competent retinal precursors. Here, through combined bioinformatic, molecular and genetic approaches in mouse retinas, we identify the Tfap2a and Tfap2b transcription factors as two major downstream effectors of Ptf1a. RNA-seq and immunolabeling analyses show that the expression of Tfap2a and 2b transcripts and proteins is dramatically downregulated in the Ptf1a null mutant retina. Their overexpression is capable of promoting the differentiation of glycinergic and GABAergic amacrine cells at the expense of photoreceptors much as misexpressed Ptf1a is, whereas their simultaneous knockdown has the opposite effect. Given the demonstrated requirement for Tfap2a and 2b in horizontal cell differentiation, our study thus defines a Foxn4/RORβ1-Ptf1a-Tfap2a/2b transcriptional regulatory cascade that underlies the competence, specification and differentiation of amacrine and horizontal cells during retinal development.

AP-2α is required after lens vesicle formation to maintain lens integrity

BACKGROUND

Transcription factors are critical in regulating lens development. The AP-2 family of transcription factors functions in differentiation, cell growth and apoptosis, and in lens and eye development. AP-2α, in particular, is important in early lens development, and when conditionally deleted at the placode stage defective separation of the lens vesicle from the surface ectoderm results. AP-2α's role during later stages of lens development is unknown. To address this, the MLR10-Cre transgene was used to delete AP-2α from the lens epithelium beginning at embryonic day (E) 10.5.

RESULTS

The loss of AP-2α after lens vesicle separation resulted in morphological defects beginning at E18.5. By P4, a small highly vacuolated lens with a multilayered epithelium was evident in the MLR10-AP-2α mutants. Epithelial cells appeared elongated and expressed fiber cell specific βB1 and γ-crystallins. Epithelial cell polarity and lens cell adhesion was disrupted and accompanied by the misexpression of ZO-1, N-Cadherin, and β-catenin. Cell death was observed in the mutant lens epithelium between postnatal day (P) 14 and P30, and correlated with altered arrangements of cells within the epithelium.

CONCLUSIONS

Our findings demonstrate that AP-2α continues to be required after lens vesicle separation to maintain a normal lens epithelial cell phenotype and overall lens integrity and to ensure correct fiber cell differentiation.

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

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

Regulation of mouse lens maturation and gene expression by Krüppel-like factor 4

Conditional disruption of Klf4 in the surface ectoderm-derived tissues of the eye results in defective cornea, conjunctiva and the lens. This report describes the effects of disruption of Klf4 in the lens in greater detail. Expression of Klf4, first detected in the embryonic day-12 (E12) mouse lens, peaked at E16 and was decreased in later stages. Early embryonic disruption of Klf4 resulted in a smaller lens with cortical vacuolation and nuclear opacity. Microarray comparison of Klf4CN and WT lens transcriptomes revealed fewer changes in the E16.5 (59 increases, 20 decreases of >1.5-fold) than the PN56 Klf4CN lens (239 increases, 182 decreases of >2-fold). Klf4-target genes in the lens were distinct from those previously identified in the cornea, suggesting disparate functions for Klf4 in these functionally related tissues. Transcripts encoding different crystallins were down-regulated in the Klf4CN lens. Shsp/αB-crystallin promoter activity was stimulated upon co-transfection with pCI-Klf4. Mitochondrial density was significantly higher in the Klf4CN lens epithelial cells, consistent with mitochondrial dysfunction being the most significantly affected pathway within the PN56 Klf4CN lens. The Klf4CN lens contained elevated levels of Alox12 and Alox15 transcripts, less reduced glutathione (GSH) and more oxidized glutathione (GSSG) than the WT, suggesting that it is oxidatively stressed. Although the expression of 2087 genes was modulated during WT lens maturation, transcripts encoding crystallins were abundant at E16.5 and remained stable at PN56. Among the 1065 genes whose expression increased during WT lens maturation, there were 104 Klf4-target genes (9.8%) with decreased expression in the PN56 Klf4CN lens. Taken together, these results demonstrate that Klf4 expression is developmentally regulated in the mouse lens, where it controls the expression of genes associated with lens maturation and redox homeostasis.

Overlapping expression patterns and redundant roles for AP-2 transcription factors in the developing mammalian retina

BACKGROUND

We have previously shown that the transcription factor AP-2α (Tcfap2a) is expressed in postmitotic developing amacrine cells in the mouse retina. Although retina-specific deletion of Tcfap2a did not affect retinogenesis, two other family members, AP-2β and AP-2γ, showed expression patterns similar to AP-2α.

RESULTS

Here we show that, in addition to their highly overlapping expression patterns in amacrine cells, AP-2α and AP-2β are also co-expressed in developing horizontal cells. AP-2γ expression is restricted to amacrine cells, in a subset that is partially distinct from the AP-2α/β-immunopositive population. To address possible redundant roles for AP-2α and AP-2β during retinogenesis, Tcfap2a/b-deficient retinas were examined. These double mutants showed a striking loss of horizontal cells and an altered staining pattern in amacrine cells that were not detected upon deletion of either family member alone.

CONCLUSIONS

These studies have uncovered critical roles for AP-2 activity in retinogenesis, delineating the overlapping expression patterns of Tcfap2a, Tcfap2b, and Tcfap2c in the neural retina, and revealing a redundant requirement for Tcfap2a and Tcfap2b in horizontal and amacrine cell development.

Analysis of TFAP2A mutations in Branchio-Oculo-Facial Syndrome indicates functional complexity within the AP-2α DNA-binding domain

Multiple lines of evidence indicate that the AP-2 transcription factor family has an important regulatory function in human craniofacial development. Notably, mutations in TFAP2A, the gene encoding AP-2α, have been identified in patients with Branchio-Oculo-Facial Syndrome (BOFS). BOFS is an autosomal-dominant trait that commonly presents with facial clefting, eye defects and branchial skin anomalies. Examination of multiple cases has suggested either simple haploinsufficiency or more complex genetic causes for BOFS, especially as the clinical manifestations are variable, with no clear genotype-phenotype correlation. Mutations occur throughout TFAP2A, but mostly within conserved sequences within the DNA contact domain of AP-2α. However, the consequences of the various mutations for AP-2α protein function have not been evaluated. Therefore, it remains unclear if all BOFS mutations result in similar changes to the AP-2α protein or if they each produce specific alterations that underlie the spectrum of phenotypes. Here, we have investigated the molecular consequences of the mutations that localize to the DNA-binding region. We show that although individual mutations have different effects on DNA binding, they all demonstrate significantly reduced transcriptional activities. Moreover, all mutant derivatives have an altered nuclear:cytoplasmic distribution compared with the predominantly nuclear localization of wild-type AP-2α and several can exert a dominant-negative activity on the wild-type AP-2α protein. Overall, our data suggest that the individual TFAP2A BOFS mutations can generate null, hypomorphic or antimorphic alleles and that these differences in activity, combined with a role for AP-2α in epigenetic events, may influence the resultant pathology and the phenotypic variability.

Ocular surface development and gene expression

The ocular surface-a continuous epithelial surface with regional specializations including the surface and glandular epithelia of the cornea, conjunctiva, and lacrimal and meibomian glands connected by the overlying tear film-plays a central role in vision. Molecular and cellular events involved in embryonic development, postnatal maturation, and maintenance of the ocular surface are precisely regulated at the level of gene expression by a well-coordinated network of transcription factors. A thorough appreciation of the biological characteristics of the ocular surface in terms of its gene expression profiles and their regulation provides us with a valuable insight into the pathophysiology of various blinding disorders that disrupt the normal development, maturation, and/or maintenance of the ocular surface. This paper summarizes the current status of our knowledge related to the ocular surface development and gene expression and the contribution of different transcription factors to this process.

Neuronal programmed cell death-1 ligand expression regulates retinal ganglion cell number in neonatal and adult mice

OBJECTIVES

During mouse retina maturation, the final number of retinal ganglion cells (RGCs) is determined by highly regulated programmed cell death. Previous studies demonstrated that the immunoregulatory receptor programmed cell death-1 (PD-1) promotes developmental RGC death. To identify the functional signaling partner(s) for PD-1, we identified retinal expression of PD-1 ligands and examined the effect of PD-1 ligand expression on RGC number. We also explored the hypothesis that PD-1 signaling promotes the development of functional visual circuitry.

METHODS

Characterization of retinal and brain programmed cell death-1 ligand 1 (PD-L1) expression were examined by immunofluorescence on tissue sections. The contribution of PD-ligands, PD-L1, and programmed cell death-1 ligand 2 (PD-L2) to RGC number was examined in PD-ligand knockout mice lacking 1 or both ligands. Retinal architecture was assessed by spectral-domain optical coherence tomography, and retinal function was analyzed by electroretinography in wild-type and PD-L1/L2 double-deficient mice.

RESULTS

PD-L1 expression is found throughout the neonatal retina and persists in adult RGCs, bipolar interneurons, and Müller glia. In the absence of both PD-ligands, there is a significant numerical increase in RGCs (34% at postnatal day 2 [P2] and 18% in adult), as compared to wild type, and PD-ligands have redundant function in this process. Despite the increased RGC number, adult PD-L1/L2 double-knockout mice have normal retinal architecture and outer retina function.

CONCLUSION

This study demonstrates that PD-L1 and PD-L2 together impact the final number of RGCs in adult mice and supports a novel role for active promotion of neuronal cell death through PD-1 receptor-ligand engagement.

In vitro organogenesis in three dimensions: self-organising stem cells

Organ formation during embryogenesis is a complex process that involves various local cell-cell interactions at the molecular and mechanical levels. Despite this complexity, organogenesis can be modelled in vitro. In this article, we focus on two recent examples in which embryonic stem cells can self-organise into three-dimensional structures - the optic cup and the pituitary epithelium; and one case of self-organising adult stem cells - the gut epithelium. We summarise how these approaches have revealed intrinsic programs that drive locally autonomous modes of organogenesis and homeostasis. We also attempt to interpret the results of previous in vivo studies of retinal development in light of the self-organising nature of the retina.

Deciphering gene expression program of MAP3K1 in mouse eyelid morphogenesis

Embryonic eyelid closure involves forward movement and ultimate fusion of the upper and lower eyelids, an essential step of mammalian ocular surface development. Although its underlying mechanism of action is not fully understood, a functional mitogen-activated protein kinase kinase kinase 1 (MAP3K1) is required for eyelid closure. Here we investigate the molecular signatures of MAP3K1 in eyelid morphogenesis. At mouse gestational day E15.5, the developmental stage immediately prior to eyelid closure, MAP3K1 expression is predominant in the eyelid leading edge (LE) and the inner eyelid (IE) epithelium. We used laser capture microdissection (LCM) to obtain highly enriched LE and IE cells from wild type and MAP3K1-deficient fetuses and analyzed genome-wide expression profiles. The gene expression data led to the identification of three distinct developmental features of MAP3K1. First, MAP3K1 modulated Wnt and Sonic hedgehog signals, actin reorganization, and proliferation only in LE but not in IE epithelium, illustrating the temporal-spatial specificity of MAP3K1 in embryogenesis. Second, MAP3K1 potentiated AP-2α expression and SRF and AP-1 activity, but its target genes were enriched for binding motifs of AP-2α and SRF, and not AP-1, suggesting the existence of novel MAP3K1-AP-2α/SRF modules in gene regulation. Third, MAP3K1 displayed variable effects on expression of lineage specific genes in the LE and IE epithelium, revealing potential roles of MAP3K1 in differentiation and lineage specification. Using LCM and expression array, our studies have uncovered novel molecular signatures of MAP3K1 in embryonic eyelid closure.

New insights into the mechanism of lens development using zebra fish

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

Ocular anomalies in an infant with Klinefelter Syndrome

Klinefelter syndrome is caused by the presence of one or more additional X chromosomes in an affected male. Patients often exhibit gynecomastia, small testes, and infertility. Though the characteristics of Klinefelter have been well-documented, associated ocular abnormalities have been only occasionally reported. Here we present a 2-month-old infant with Klinefelter syndrome and a unique combination of ocular abnormalities including microphthalmia, cataracts, and malformed pupils.

Requirements for Jag1-Rbpj mediated Notch signaling during early mouse lens development

BACKGROUND

During vertebrate lens development, the lens placode in the embryonic ectoderm invaginates into a lens vesicle, which then separates from the surface epithelium, followed by two waves of fiber cell differentiation. In the mouse, multiple labs have shown that Jag1-Notch signaling is critically required during the second wave of lens fiber cell formation. However, Notch signaling appears to play no obvious role during lens induction or morphogenesis, although multiple pathway genes are expressed at these earlier stages.

RESULTS

Here, we explored functions for Notch signaling specifically during early lens development, by using the early-acting AP2α-Cre driver to delete Jag1 or Rbpj. We found that Jag1 and Rbpj are not required during lens induction, but are necessary for proper lens vesicle separation from the surface ectoderm.

CONCLUSIONS

We conclude that precise levels of Notch signaling are essential during lens vesicle morphogenesis. In addition, AP2α-Cre-mediated deletion of Rbpj resulted in embryos with cardiac outflow tract and liver deformities, and perinatal lethality.

Relaxation-expansion model for self-driven retinal morphogenesis: a hypothesis from the perspective of biosystems dynamics at the multi-cellular level

The generation of complex organ structures such as the eye requires the intricate orchestration of multiple cellular interactions. In this paper, early retinal development is discussed with respect to the structure formation of the optic cup. Although recent studies have elucidated molecular mechanisms of retinal differentiation, little is known about how the unique shape of the optic cup is determined. A recent report has demonstrated that optic-cup morphogenesis spontaneously occurs in three-dimensional stem-cell culture without external forces, indicating a latent intrinsic order to generate the structure. Based on this self-organizing phenomenon, we introduce the "relaxation-expansion" model to mechanically interpret the tissue dynamics that enable the spontaneous invagination of the neural retina. This model involves three consecutive local rules (relaxation, apical constriction, and expansion), and its computer simulation recapitulates the optic-cup morphogenesis in silico.

A family with branchio-oculo-facial syndrome with primarily ocular involvement associated with mutation of the TFAP2A gene

BACKGROUND

Branchio-Oculo-Facial syndrome (BOFS) is a rare, autosomal dominant developmental disorder that has a distinct phenotype with characteristic craniofacial abnormalities. We report a family with extensive ocular manifestations of BOFS caused by a novel mutation in the transcription factor AP-2 alpha (TFAP2A) gene.

MATERIALS AND METHODS

Case report of phenotypic and genotypic characterization of a family with BOFS.

RESULTS

An infant presenting with anophththalmia/coloboma and subtle craniofacial symptoms was found to have a family history of congenital cataracts and colobomas in her mother. A mutation in the TFAP2A gene associated with BOFS (heterozygous H384Y in exon 7) was found in both the proband and her mother. This mutation had not been reported previously. Compared with other molecularly confirmed cases in the literature, this family has primarily ocular features, which are severe.

CONCLUSIONS

BOFS can have profound ocular involvement without prominent extraocular features. When the syndrome presents in this way, it may be confused with isolated autosomal dominant chorioretinal coloboma. Testing for mutations in the TFAP2A gene is recommended to establish an accurate diagnosis for the family.

A miR-200b/200c/429-binding site polymorphism in the 3' untranslated region of the AP-2α gene is associated with cisplatin resistance

The transcription factor AP-2α functions as a tumor suppressor by regulating various genes that are involved in cell proliferation and apoptosis. Chemotherapeutic drugs including cisplatin induce post-transcriptionally endogenous AP-2α, which contributes to chemosensitivity by enhancing therapy-induced apoptosis. microRNAs (miRNAs) miR-200b, miR-200c and miR-429 (miR-200b/200c/429) are up-regulated in endometrial and esophageal cancers, and their overexpression correlates with resistance to cisplatin treatment. Using computational programs, we predicted that the 3′ untranslated region (UTR) of AP-2α gene contains a potential miRNA response element (MRE) for the miR-200b/200c/429 family, and the single nucleotide polymorphism (SNP) site rs1045385 (A or C allele) resided within the predicted MRE. Luciferase assays and Western blot analysis demonstrated that the miR-200b/200c/429 family recognized the MRE in the 3′ UTR of AP-2α gene and negatively regulated the expression of endogenous AP-2α proteins. SNP rs1045385 A>C variation enhanced AP-2α expression by disrupting the binding of the miR-200b/200c/429 family to the 3′ UTR of AP-2α. The effects of the two polymorphic variants on cisplatin sensitivity were determined by clonogenic assay. The overexpression of AP-2α with mutant 3′ UTR (C allele) in the endometrial cancer cell line HEC-1A, which has high levels of endogenous miR-200b/200c/429 and low levels of AP-2α protein, significantly increased cisplatin sensitivity, but overexpression of A allele of AP-2α has no significant effects, compared with mock transfection. We concluded that miR-200b/200c/429 induced cisplatin resistance by repressing AP-2α expression in endometrial cancer cells. The SNP (rs1045385) A>C variation decreased the binding of miR-200b/200c/429 to the 3′ UTR of AP-2α, which upregulated AP-2α protein expression and increased cisplatin sensitivity. Our results suggest that SNP (rs1045385) may be a potential prognostic marker for cisplatin treatment.

A novel TFAP2A mutation in familial Branchio-Oculo-Facial Syndrome with predominant ocular phenotype

INTRODUCTION

Branchio-Oculo-Facial Syndrome (BOFS) is a rare autosomal dominant congenital disorder defined by branchial defects, ocular anomalies and craniofacial malformations, including variable degrees of cleft lip with or without cleft palate. In addition, temporal bone anomalies, renal and ectodermal manifestations can be present. Mutations in the TFAP2A gene have been reported in patients with BOFS, prompting phenotype-genotype studies because of the variable clinical spectrum.

MATERIALS AND METHODS

We report on a family (a mother, her daughter and son) with BOFS and significant variability in clinical expression. The daughter presents predominantly with an ocular phenotype of unilateral microphthalmia and bilateral chorioretinal colobomas, whereas her brother is more severely affected contrasting with the paucisymptomatic mother. TFAP2A molecular analysis revealed a novel frameshift mutation.

DISCUSSION

We confirm the wide clinical spectrum of BOFS. The importance of upper lip examination in mild and paucisymptomatic cases is underlined. TFAP2A mutation spectrum is discussed and broadened by the report of the second frameshift mutation in this gene.

CONCLUSION

Patients with BOFS and predominant ocular phenotypes can be underdiagnosed. In such cases, upper lip examination can be of important diagnostic value. TFAP2A analysis provides diagnostic confirmation and improves genetic counselling.

Transcriptomics for understanding marine fish larval development1This review is part of a virtual symposium on current topics in aquaculture of marine fish and shellfish

The larval phase is a crucial period in the life of marine fish. During this phase, the organism will acquire the phenotype of an adult fish through the development of tissues and organs and the maturation of some of the principal physiological functions. Many biological processes (differentiation, cellular proliferation, growth, etc.) are regulated during this period. These regulations take place at different biological levels and particularly concern the expression of genes involved in larval ontogenesis processes. The development of bioinformatic resources (DNA or cDNA sequences) and molecular tools enabling high throughput gene expression analysis (microarrays) have allowed the transcriptome of marine fish species to be studied. In the present review, we summarize the main findings from transcriptomic investigations of development of marine fish larvae. Special attention is paid to investigations of transcriptomic patterns during postembryonic development and to the impact of environmental or nutritional factors on the transcriptome of marine fish larvae. Transcriptomic approaches will be especially useful in the future for investigating the effect of temperature and water acidification (or pH) on the development of different fish species in the context of global climate change.

Aberrant expressions of AP-2α splice variants in pancreatic cancer

OBJECTIVES

The present study was conducted to evaluate the expression and function of AP-2α isoforms in pancreatic ductal adenocarcinoma.

METHODS

The expression of AP-2α was evaluated at the RNA level by reverse transcription-polymerase chain reaction and at the protein level by Western blotting and immunofluorescence. Its function as a transcription factor was evaluated in transient transfection experiments: DNA binding properties by electromobility shift assay and transactivation capabilities by luciferase assay.

RESULTS

Multiple alternative splicing events of AP-2α messenger occurred in all human pancreatic cancer cell lines, including a novel isoform, termed variant 6, which was not present in HeLa cells. At the protein level, except for 1 cell line, all pancreatic cancer cell lines expressed high nuclear levels of AP-2α. We also showed that AP-2α expressed by the pancreatic cancer cell lines could bind its cognate recognition site and activate transcription. However, variant 6, although not able to activate transcription, did not act in a dominant negative manner when cotransfected with the full-length protein.

CONCLUSIONS

Multiple isoforms of AP-2α are highly expressed in pancreatic cancer cell lines including a new isoform, AP-2α variant 6, which seems to be pancreatic cancer specific and is deprived of transcriptional activity.

MIP/Aquaporin 0 represents a direct transcriptional target of PITX3 in the developing lens

The PITX3 bicoid-type homeodomain transcription factor plays an important role in lens development in vertebrates. PITX3 deficiency results in a spectrum of phenotypes from isolated cataracts to microphthalmia in humans, and lens degeneration in mice and zebrafish. While identification of downstream targets of PITX3 is vital for understanding the mechanisms of normal ocular development and human disease, these targets remain largely unknown. To isolate genes that are directly regulated by PITX3, we performed a search for genomic sequences that contain evolutionarily conserved bicoid/PITX3 binding sites and are located in the proximity of known genes. Two bicoid sites that are conserved from zebrafish to human were identified within the human promoter of the major intrinsic protein of lens fiber, MIP/AQP0. MIP/AQP0 deficiency was previously shown to be associated with lens defects in humans and mice. We demonstrate by both chromatin immunoprecipitation and electrophoretic mobility shift assay that PITX3 binds to MIP/AQP0 promoter region in vivo and is able to interact with both bicoid sites in vitro. In addition, we show that wild-type PITX3 is able to activate the MIP/AQP0 promoter via interaction with the proximal bicoid site in cotransfection experiments and that the introduction of mutations disrupting binding to this site abolishes this activation. Furthermore, mutant forms of PITX3 fail to produce the same levels of transactivation as wild-type when cotransfected with the MIP/AQP0 reporter. Finally, knockdown of pitx3 in zebrafish affects formation of a DNA-protein complex associated with mip1 promoter sequences; and examination of expression in pitx3 morphant and control zebrafish revealed a delay in and reduction of mip1 expression in pitx3-deficient embryos. Therefore, our data suggest that PITX3 is involved in direct regulation of MIP/AQP0 expression and that the alteration of MIP/AQP0 expression is likely to contribute to the lens phenotype in cataract patients with PITX3 mutations.

Reduced TFAP2A function causes variable optic fissure closure and retinal defects and sensitizes eye development to mutations in other morphogenetic regulators

Mutations in the transcription factor encoding TFAP2A gene underlie branchio-oculo-facial syndrome (BOFS), a rare dominant disorder characterized by distinctive craniofacial, ocular, ectodermal and renal anomalies. To elucidate the range of ocular phenotypes caused by mutations in TFAP2A, we took three approaches. First, we screened a cohort of 37 highly selected individuals with severe ocular anomalies plus variable defects associated with BOFS for mutations or deletions in TFAP2A. We identified one individual with a de novo TFAP2A four amino acid deletion, a second individual with two non-synonymous variations in an alternative splice isoform TFAP2A2, and a sibling-pair with a paternally inherited whole gene deletion with variable phenotypic expression. Second, we determined that TFAP2A is expressed in the lens, neural retina, nasal process, and epithelial lining of the oral cavity and palatal shelves of human and mouse embryos--sites consistent with the phenotype observed in patients with BOFS. Third, we used zebrafish to examine how partial abrogation of the fish ortholog of TFAP2A affects the penetrance and expressivity of ocular phenotypes due to mutations in genes encoding bmp4 or tcf7l1a. In both cases, we observed synthetic, enhanced ocular phenotypes including coloboma and anophthalmia when tfap2a is knocked down in embryos with bmp4 or tcf7l1a mutations. These results reveal that mutations in TFAP2A are associated with a wide range of eye phenotypes and that hypomorphic tfap2a mutations can increase the risk of developmental defects arising from mutations at other loci.

Lens induction requires attenuation of ERK signaling by Nf1

Aphakia (lack of lens) is a rare human congenital disorder with its genetic etiology largely unknown. Even in model organisms, very few mutations are known to result in such a drastic ocular defect. In this study, we have shown that homozygous deletion of Nf1, the Ras GTPase gene underlying human neurofibromatosis type 1 syndrome, causes lens dysgenesis in mouse. Although early lens specification proceeded normally in Nf1 mutants, lens induction was disrupted due to deficient cell proliferation. Further analysis showed that extracellular signal-regulated kinase (ERK) signaling was initially elevated in the invaginating lens placode, but by the lens vesicle stage, ERK phosphorylation was significantly reduced. Only after intraperitoneal treatment of U0126, an inhibitor of ERK phosphorylation, was lens development restored in Nf1 mutants. Hyperactive Ras-mitogen-activated protein kinase (MAPK) signaling is known to cause neuro-cardiofacial-cutaneous (NCFC) syndromes in humans. As a member of NCFC family genes, Nf1 represents the first example that attenuation of Ras-MAPK kinase signaling pathway is essential for normal lens development.