The role of Pax-6 in eye and nasal development

Homeodomain-interacting protein kinase phosphorylates the Drosophila Paired box protein 6 (Pax6) homologues Twin of eyeless and Eyeless

Homeodomain-interacting protein kinase (Hipk), the Drosophila homologue of mammalian HIPK2, plays several important roles in regulating differentiation, proliferation, apoptosis, and stress responses and acts as a mediator for signals of diverse pathways, such as Notch or Wingless signalling. The Paired box protein 6 (Pax6) has two Drosophila homologues, Twin of eyeless (Toy) and Eyeless (Ey). Both stand atop the retinal determination gene network (RDGN), which is essential for proper eye development in Drosophila. Here, we set Hipk and the master regulators Toy and Ey in an enzyme-substrate relationship. Furthermore, we prove a physical interaction between Toy and Hipk in vivo using bimolecular fluorescence complementation. Using in vitro kinase assays with different truncated Toy constructs and mutational analyses, we mapped four Hipk phosphorylation sites of Toy, one in the paired domain (Ser¹²¹ ) and three in the C-terminal transactivation domain of Toy (Thr³⁹⁵ , Ser⁴¹⁰ and Thr⁴⁵² ). The interaction and phosphorylation of the master regulator Toy by Hipk may be important for precise tuning of signalling within the RDGN and therefore for Drosophila eye development.

Lhx2, an evolutionarily conserved, multifunctional regulator of forebrain development

A hundred years after Lhx2 ortholog apterous was identified as a critical regulator of wing development in Drosophila, LIM-HD gene family members have proved to be versatile and powerful components of the molecular machinery that executes the blueprint of embryogenesis across vertebrate and invertebrate species. Here, we focus on the spatio-temporally varied functions of LIM-homeodomain transcription factor LHX2 in the developing mouse forebrain. Right from its earliest known role in telencephalic and eye field patterning, to the control of the neuron-glia cell fate switch, and the regulation of axon pathfinding and dendritic arborization in late embryonic stages, LHX2 has been identified as a fundamental, temporally dynamic, always necessary, and often sufficient factor in a range of critical developmental phenomena. While Lhx2 mutant phenotypes have been characterized in detail in multiple brain structures, only recently have we advanced in our understanding of the molecular mechanisms by which this factor acts. Common themes emerge from how this multifunctional molecule controls a range of developmental steps in distinct forebrain structures. Examining these shared features, and noting unique aspects of LHX2 function is likely to inform our understanding of how a single factor can bring about a diversity of effects and play central and critical roles across systems and stages. The parallels in LHX2 and APTEROUS functions, and the protein complexes they participate in, offer insights into evolutionary strategies that conserve tool kits and deploy them to play new, yet familiar roles in species separated by hundreds of millions of years.

Novel functions of LHX2 and PAX6 in the developing telencephalon revealed upon combined loss of both genes

Patterning of the telencephalic neuroepithelium is a tightly regulated process controlled by transcription factors and signalling molecules. The cortical primordium is flanked by two signalling centres, the hem medially, and the antihem laterally. The hem induces the formation of the hippocampus in adjacent neuroepithelium. Therefore, the position of the hem defines the position of the hippocampus in the brain. The antihem is positioned at the boundary between the dorsal and ventral telencephalon and proposed to provide patterning cues during development.

LIM-homeodomain (LIM-HD) transcription factor LHX2 suppresses both hem and antihem fate in the cortical neuroepithelium. Upon loss of Lhx2, medial cortical neuroepithelium is transformed into hem, whereas lateral cortical neuroepithelium is transformed into antihem. Here, we show that transcription factor PAX6, known to regulate patterning of the lateral telencephalon, restricts this tissue from transforming into hem upon loss of Lhx2. When Lhx2 and Pax6 are both deleted, the cortical hem expands to occupy almost the complete extent of the cortical primordium, indicating that both factors act to suppress hem fate in the lateral telencephalon. Furthermore, the shift in the pallial-subpallial boundary and absence of the antihem, observed in the Pax6 mutant, are both restored in the Lhx2; Pax6 double mutant.

Together, these results not only reveal a novel function for LHX2 in regulating dorsoventral patterning in the telencephalon, but also identify PAX6 as a fundamental regulator of where the hem can form, and therefore implicate this molecule as a determinant of hippocampal positioning.

Limbal stem cells: identity, developmental origin, and therapeutic potential

The cornea is our window to the world and our vision is critically dependent on corneal clarity and integrity. Its epithelium represents one of the most rapidly regenerating mammalian tissues, undergoing full-turnover over the course of approximately 1-2 weeks. This robust and efficient regenerative capacity is dependent on the function of stem cells residing in the limbus, a structure marking the border between the cornea and the conjunctiva. Limbal stem cells (LSC) represent a quiescent cell population with proliferative capacity residing in the basal epithelial layer of the limbus within a cellular niche. In addition to LSC, this niche consists of various cell populations such as limbal stromal fibroblasts, melanocytes and immune cells as well as a basement membrane, all of which are essential for LSC maintenance and LSC-driven regeneration. The LSC niche's components are of diverse developmental origin, a fact that had, until recently, prevented precise identification of molecularly defined LSC. The recent success in prospective LSC isolation based on ABCB5 expression and the capacity of this LSC population for long-term corneal restoration following transplantation in preclinical in vivo models of LSC deficiency underline the considerable potential of pure LSC formulations for clinical therapy. Additional studies, including genetic lineage tracing of the developmental origin of LSC will further improve our understanding of this critical cell population and its niche, with important implications for regenerative medicine. WIREs Dev Biol 2018, 7:e303. doi: 10.1002/wdev.303 This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cells and Disease Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration.

Cell Cycle-Related Kinase (CCRK) regulates ciliogenesis and Hedgehog signaling in mice

The Hedgehog (Hh) signaling pathway plays a key role in cell fate specification, proliferation, and survival during mammalian development. Cells require a small organelle, the primary cilium, to respond properly to Hh signals and the key regulators of Hh signal transduction exhibit dynamic localization to this organelle when the pathway is activated. Here, we investigate the role of Cell Cycle Related kinase (CCRK) in regulation of cilium-dependent Hh signaling in the mouse. Mice mutant for Ccrk exhibit a variety of developmental defects indicative of inappropriate regulation of this pathway. Cell biological, biochemical and genetic analyses indicate that CCRK is required to control the Hedgehog pathway at the level or downstream of Smoothened and upstream of the Gli transcription factors, Gli2 and Gli3. In vitro experiments indicate that Ccrk mutant cells show a greater deficit in response to signaling over long time periods than over short ones. Similar to Chlamydomonas mutants lacking the CCRK homolog, LF2, mouse Ccrk mutant cells show defective regulation of ciliary length and morphology. Ccrk mutant cells exhibit defects in intraflagellar transport (the transport mechanism used to assemble cilia), as well as slowed kinetics of ciliary enrichment of key Hh pathway regulators. Collectively, the data suggest that CCRK positively regulates the kinetics by which ciliary proteins such as Smoothened and Gli2 are imported into the cilium, and that the efficiency of ciliary recruitment allows for potent responses to Hedgehog signaling over long time periods.

The importance of cilia in development and disease has become broadly appreciated in recent years due in part to their roles in signal transduction. Despite this attention, crucial aspects of ciliary assembly and function, such as the mechanisms controlling ciliary assembly and the signal transduction events occurring in cilia, remain unclear. Cilia play a central role in sensing and transducing Hedgehog signals in the context of mammalian embryogenesis and in a variety of cancers. Here, we investigate the functions of Cell Cycle Related Kinase (CCRK), which plays an evolutionarily conserved function in the assembly of cilia and flagella. We find that mouse CCRK positively and negatively regulates ciliary length. We show that CCRK controls multiple aspects of Hedgehog signaling in vivo and in vitro by regulating the processing and activities of the Gli transcription factors. Our data suggest that CCRK controls Hedgehog signaling by promoting the efficient ciliary import of core signaling components.

Association of SIX1/SIX6 locus polymorphisms with regional circumpapillary retinal nerve fibre layer thickness: The Nagahama study

SIX1 and SIX6 are glaucoma susceptibility genes. Previous reports indicate that the single nucleotide polymorphism (SNP) rs33912345 in SIX6 is associated with inferior circumpapillary retinal nerve fibre layer (cpRNFL) thickness (cpRNFLT). Although the region of visual field defect in glaucoma patients is directly related to cpRNFL thinning, a detailed sector analysis has not been performed in genetic association studies. In the present study, we evaluated 26 tagging SNPs in the SIX1/SIX6 locus ±50 kb region in a population of 2,306 Japanese subjects with 4- and 32-sector cpRNFLT analysis. While no SNPs showed a significant association with cpRNFLT in the 4-sectored analysis, the finer 32-sector assessment clearly showed a significant association between rs33912345 in the SIX1/SIX6 locus with inferior cpRNFL thinning at 292.5-303.8° (β = -4.55, P = 3.0 × 10^-5). Furthermore, the fine-sectored cpRNFLT analysis indicated that SIX1/SIX6 polymorphisms would affect cpRNFL thinning at 281.3-303.8°, which corresponds to parafoveal scotoma in a visual field test of glaucoma patients.

Lateral thinking - Interocular symmetry and asymmetry in neurovascular patterning, in health and disease

No biological system or structure is likely to be perfectly symmetrical, or have identical right and left forms. This review explores the evidence for eye and visual pathway asymmetry, in health and in disease, and attempts to provide guidance for those studying the structure and function of the visual system, where recognition of symmetry or asymmetry may be essential. The principal question with regards to asymmetry is not 'are the eyes the same?', for some degree of asymmetry is pervasive, but 'when are they importantly different?'. Knowing if right and left eyes are 'importantly different' could have significant consequences for deciding whether right or left eyes are included in an analysis or for examining the association between a phenotype and ocular parameter. The presence of significant asymmetry would also have important implications for the design of normative databases of retinal and optic nerve metrics. In this review, we highlight not only the universal presence of asymmetry, but provide evidence that some elements of the visual system are inherently more asymmetric than others, pointing to the need for improved normative data to explain sources of asymmetry and their impact on determining associations with genetic, environmental or health-related factors and ultimately in clinical practice.

Foxd4 is essential for establishing neural cell fate and for neuronal differentiation

Many molecular factors required for later stages of neuronal differentiation have been identified; however, much less is known about the early events that regulate the initial establishment of the neuroectoderm. We have used an in vitro embryonic stem cell (ESC) differentiation model to investigate early events of neuronal differentiation and to define the role of mouse Foxd4, an ortholog of a forkhead-family transcription factor central to Xenopus neural plate/neuroectodermal precursor development. We found that Foxd4 is a necessary regulator of the transition from pluripotent ESC to neuroectodermal stem cell, and its expression is necessary for neuronal differentiation. Mouse Foxd4 expression is not only limited to the neural plate but it is also expressed and apparently functions to regulate neurogenesis in the olfactory placode. These in vitro results suggest that mouse Foxd4 has a similar function to its Xenopus ortholog; this was confirmed by successfully substituting murine Foxd4 for its amphibian counterpart in overexpression experiments. Thus, Foxd4 appears to regulate the initial steps in establishing neuroectodermal precursors during initial development of the nervous system.

Relationship between somatic mosaicism of Pax6 mutation and variable developmental eye abnormalities-an analysis of CRISPR genome-edited mouse embryos

The clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (Cas) system is a rapid gene-targeting technology that does not require embryonic stem cells. To demonstrate dosage effects of the Pax6 gene on eye formation, we generated Pax6-deficient mice with the CRISPR/Cas system. Eyes of founder embryos at embryonic day (E) 16.5 were examined and categorized according to macroscopic phenotype as class 1 (small eye with distinct pigmentation), class 2 (pigmentation without eye globes), or class 3 (no pigmentation and no eyes). Histologically, class 1 eyes were abnormally small in size with lens still attached to the cornea at E16.5. Class 2 eyes had no lens and distorted convoluted retinas. Class 3 eyes had only rudimentary optic vesicle-like tissues or histological anophthalmia. Genotyping of neck tissue cells from the founder embryos revealed somatic mosaicism and allelic complexity for Pax6. Relationships between eye phenotype and genotype were developed. The present results demonstrated that development of the lens from the surface ectoderm requires a higher gene dose of Pax6 than development of the retina from the optic vesicle. We further anticipate that mice with somatic mosaicism in a targeted gene generated by CRISPR/Cas-mediated genome editing will give some insights for understanding the complexity in human congenital diseases that occur in mosaic form.

Temporal dissociation of developmental events in the chick eye under low temperature conditions

The chick embryonic eye is an excellent model for the study of vertebrate organogenesis. Key events in eye development involve thickening, invagination and cytodifferentiation of the lens primordium. While these events occur successively at different developmental stages, the extent to which these events are temporally related is largely unknown. Here we show that the lens invagination is highly sensitive to temperature. Lowering of incubation temperature to 29°C at embryonic day 2 delayed the onset of invagination of the lens, but not thickening and cytodifferentiation, leading to abnormal protrusion of the eye. The temperature shift also delayed the inward bending of the underlying retinal primordium, even in the absence of the lens. Taken together, our results suggest that lens invagination is initiated independently of thickening and cytodifferentiation, possibly by mechanisms associated with morphogenesis of the primordial retina.

The Gene Regulatory Network of Lens Induction Is Wired through Meis-Dependent Shadow Enhancers of Pax6

Lens induction is a classical developmental model allowing investigation of cell specification, spatiotemporal control of gene expression, as well as how transcription factors are integrated into highly complex gene regulatory networks (GRNs). Pax6 represents a key node in the gene regulatory network governing mammalian lens induction. Meis1 and Meis2 homeoproteins are considered as essential upstream regulators of Pax6 during lens morphogenesis based on their interaction with the ectoderm enhancer (EE) located upstream of Pax6 transcription start site. Despite this generally accepted regulatory pathway, Meis1-, Meis2- and EE-deficient mice have surprisingly mild eye phenotypes at placodal stage of lens development. Here, we show that simultaneous deletion of Meis1 and Meis2 in presumptive lens ectoderm results in arrested lens development in the pre-placodal stage, and neither lens placode nor lens is formed. We found that in the presumptive lens ectoderm of Meis1/Meis2 deficient embryos Pax6 expression is absent. We demonstrate using chromatin immunoprecipitation (ChIP) that in addition to EE, Meis homeoproteins bind to a remote, ultraconserved SIMO enhancer of Pax6. We further show, using in vivo gene reporter analyses, that the lens-specific activity of SIMO enhancer is dependent on the presence of three Meis binding sites, phylogenetically conserved from man to zebrafish. Genetic ablation of EE and SIMO enhancers demostrates their requirement for lens induction and uncovers an apparent redundancy at early stages of lens development. These findings identify a genetic requirement for Meis1 and Meis2 during the early steps of mammalian eye development. Moreover, they reveal an apparent robustness in the gene regulatory mechanism whereby two independent "shadow enhancers" maintain critical levels of a dosage-sensitive gene, Pax6, during lens induction.

WNT/β-Catenin Signaling in Vertebrate Eye Development

The vertebrate eye is a highly specialized sensory organ, which is derived from the anterior neural plate, head surface ectoderm, and neural crest-derived mesenchyme. The single central eye field, generated from the anterior neural plate, divides to give rise to the optic vesicle, which evaginates toward the head surface ectoderm. Subsequently, the surface ectoderm, in conjunction with the optic vesicle invaginates to form the lens vesicle and double-layered optic cup, respectively. This complex process is controlled by transcription factors and several intracellular and extracellular signaling pathways including WNT/β-catenin signaling. This signaling pathway plays an essential role in multiple developmental processes and has a profound effect on cell proliferation and cell fate determination. During eye development, the activity of WNT/β-catenin signaling is tightly controlled. Faulty regulation of WNT/β-catenin signaling results in multiple ocular malformations due to defects in the process of cell fate determination and differentiation. This mini-review summarizes recent findings on the role of WNT/β-catenin signaling in eye development. Whilst this mini-review focuses on loss-of-function and gain-of-function mutants of WNT/β-catenin signaling components, it also highlights some important aspects of β-catenin-independent WNT signaling in the eye development at later stages.

Pax6 associates with H3K4-specific histone methyltransferases Mll1, Mll2, and Set1a and regulates H3K4 methylation at promoters and enhancers

BACKGROUND

Pax6 is a key regulator of the entire cascade of ocular lens formation through specific binding to promoters and enhancers of batteries of target genes. The promoters and enhancers communicate with each other through DNA looping mediated by multiple protein-DNA and protein-protein interactions and are marked by specific combinations of histone posttranslational modifications (PTMs). Enhancers are distinguished from bulk chromatin by specific modifications of core histone H3, including H3K4me1 and H3K27ac, while promoters show increased H3K4me3 PTM. Previous studies have shown the presence of Pax6 in as much as 1/8 of lens-specific enhancers but a much smaller fraction of tissue-specific promoters. Although Pax6 is known to interact with EP300/p300 histone acetyltransferase responsible for generation of H3K27ac, a potential link between Pax6 and histone H3K4 methylation remains to be established.

RESULTS

Here we show that Pax6 co-purifies with H3K4 methyltransferase activity in lens cell nuclear extracts. Proteomic studies show that Pax6 immunoprecipitates with Set1a, Mll1, and Mll2 enzymes, and their associated proteins, i.e., Wdr5, Rbbp5, Ash2l, and Dpy30. ChIP-seq studies using chromatin prepared from mouse lens and cultured lens cells demonstrate that Pax6-bound regions are mostly enriched with H3K4me2 and H3K4me1 in enhancers and promoters, though H3K4me3 marks only Pax6-containing promoters. The shRNA-mediated knockdown of Pax6 revealed down-regulation of a set of direct target genes, including Cap2, Farp1, Pax6, Plekha1, Prox1, Tshz2, and Zfp536. Pax6 knockdown was accompanied by reduced H3K4me1 at enhancers and H3K4me3 at promoters, with little or no changes of the H3K4me2 modifications. These changes were prominent in Plekha1, a gene regulated by Pax6 in both lens and retinal pigmented epithelium.

CONCLUSIONS

Our study supports a general model of Pax6-mediated recruitment of histone methyltransferases Mll1 and Mll2 to lens chromatin, especially at distal enhancers. Genome-wide data in lens show that Pax6 binding correlates with H3K4me2, consistent with the idea that H3K4me2 PTMs correlate with the binding of transcription factors. Importantly, partial reduction of Pax6 induces prominent changes in local H3K4me1 and H3K4me3 modification. Together, these data open the field to mechanistic studies of Pax6, Mll1, Mll2, and H3K4me1/2/3 dynamics at distal enhancers and promoters of developmentally controlled genes.

mSWI/SNF (BAF) Complexes Are Indispensable for the Neurogenesis and Development of Embryonic Olfactory Epithelium

Neurogenesis is a key developmental event through which neurons are generated from neural stem/progenitor cells. Chromatin remodeling BAF (mSWI/SNF) complexes have been reported to play essential roles in the neurogenesis of the central nervous system. However, whether BAF complexes are required for neuron generation in the olfactory system is unknown. Here, we identified onscBAF and ornBAF complexes, which are specifically present in olfactory neural stem cells (oNSCs) and olfactory receptor neurons (ORNs), respectively. We demonstrated that BAF155 subunit is highly expressed in both oNSCs and ORNs, whereas high expression of BAF170 subunit is observed only in ORNs. We report that conditional deletion of BAF155, a core subunit in both onscBAF and ornBAF complexes, causes impaired proliferation of oNSCs as well as defective maturation and axonogenesis of ORNs in the developing olfactory epithelium (OE), while the high expression of BAF170 is important for maturation of ORNs. Interestingly, in the absence of BAF complexes in BAF155/BAF170 double-conditional knockout mice (dcKO), OE is not specified. Mechanistically, BAF complex is required for normal activation of Pax6-dependent transcriptional activity in stem cells/progenitors of the OE. Our findings unveil a novel mechanism mediated by the mSWI/SNF complex in OE neurogenesis and development.

Hedgehog-dependent E3-ligase Midline1 regulates ubiquitin-mediated proteasomal degradation of Pax6 during visual system development

Pax6 is a key transcription factor involved in eye, brain, and pancreas development. Although pax6 is expressed in the whole prospective retinal field, subsequently its expression becomes restricted to the optic cup by reciprocal transcriptional repression of pax6 and pax2 However, it remains unclear how Pax6 protein is removed from the eyestalk territory on time. Here, we report that Mid1, a member of the RBCC/TRIM E3 ligase family, which was first identified in patients with the X-chromosome-linked Opitz BBB/G (OS) syndrome, interacts with Pax6. We found that the forming eyestalk is a major domain of mid1 expression, controlled by the morphogen Sonic hedgehog (Shh). Here, Mid1 regulates the ubiquitination and proteasomal degradation of Pax6 protein. Accordantly, when Mid1 levels are knocked down, Pax6 expression is expanded and eyes are enlarged. Our findings indicate that remaining or misaddressed Pax6 protein is cleared from the eyestalk region to properly set the border between the eyestalk territory and the retina via Mid1. Thus, we identified a posttranslational mechanism, regulated by Sonic hedgehog, which is important to suppress Pax6 activity and thus breaks pax6 autoregulation at defined steps during the formation of the visual system.

PAX6 MiniPromoters drive restricted expression from rAAV in the adult mouse retina

Current gene therapies predominantly use small, strong, and readily available ubiquitous promoters. However, as the field matures, the availability of small, cell-specific promoters would be greatly beneficial. Here we design seven small promoters from the human paired box 6 (PAX6) gene and test them in the adult mouse retina using recombinant adeno-associated virus. We chose the retina due to previous successes in gene therapy for blindness, and the PAX6 gene since it is: well studied; known to be driven by discrete regulatory regions; expressed in therapeutically interesting retinal cell types; and mutated in the vision-loss disorder aniridia, which is in need of improved therapy. At the PAX6 locus, 31 regulatory regions were bioinformatically predicted, and nine regulatory regions were constructed into seven MiniPromoters. Driving Emerald GFP, these MiniPromoters were packaged into recombinant adeno-associated virus, and injected intravitreally into postnatal day 14 mice. Four MiniPromoters drove consistent retinal expression in the adult mouse, driving expression in combinations of cell-types that endogenously express Pax6: ganglion, amacrine, horizontal, and Müller glia. Two PAX6-MiniPromoters drive expression in three of the four cell types that express PAX6 in the adult mouse retina. Combined, they capture all four cell types, making them potential tools for research, and PAX6-gene therapy for aniridia.

A Novel Homozygous Mutation in FOXC1 Causes Axenfeld Rieger Syndrome with Congenital Glaucoma

BACKGROUND

Anterior segment dysgenesis (ASD) disorders are a group of clinically and genetically heterogeneous phenotypes in which frequently cornea, iris, and lens are affected. This study aimed to identify novel mutations in PAX6, PITX2 and FOXC1 in families with anterior segment dysgenesis disorders.

METHODS

We studied 14 Pakistani and one Mexican family with Axenfeld Rieger syndrome (ARS; n = 10) or aniridia (n = 5). All affected and unaffected family members underwent full ophthalmologic and general examinations. Total genomic DNA was isolated from peripheral blood. PCR and Sanger sequencing were performed for the exons and intron-exon boundaries of the FOXC1, PAX6, and PITX2 genes.

RESULTS

Mutations were identified in five of the 15 probands; four variants were novel and one variant was described previously. A novel de novo variant (c.225C>A; p.Tyr75*) was identified in the PAX6 gene in two unrelated probands with aniridia. In addition, a known variant (c.649C>T; p.Arg217*) in PAX6 segregated in a family with aniridia. In the FOXC1 gene, a novel heterozygous variant (c.454T>C; p.Trp152Arg) segregated with the disease in a Mexican family with ARS. A novel homozygous variant (c.92_100del; p.Ala31_Ala33del) in the FOXC1 gene segregated in a Pakistani family with ARS and congenital glaucoma.

CONCLUSIONS

Our study expands the mutation spectrum of the PAX6 and FOXC1 genes in individuals with anterior segment dysgenesis disorders. In addition, our study suggests that FOXC1 mutations, besides typical autosomal dominant ARS, can also cause ARS with congenital glaucoma through an autosomal recessive inheritance pattern. Our results thus expand the disease spectrum of FOXC1, and may lead to a better understanding of the role of FOXC1 in development.

Differentiation/Purification Protocol for Retinal Pigment Epithelium from Mouse Induced Pluripotent Stem Cells as a Research Tool

Purpose

To establish a novel protocol for differentiation of retinal pigment epithelium (RPE) with high purity from mouse induced pluripotent stem cells (iPSC).

Methods

Retinal progenitor cells were differentiated from mouse iPSC, and RPE differentiation was then enhanced by activation of the Wnt signaling pathway, inhibition of the fibroblast growth factor signaling pathway, and inhibition of the Rho-associated, coiled-coil containing protein kinase signaling pathway. Expanded pigmented cells were purified by plate adhesion after Accutase^® treatment. Enriched cells were cultured until they developed a cobblestone appearance with cuboidal shape. The characteristics of iPS-RPE were confirmed by gene expression, immunocytochemistry, and electron microscopy. Functions and immunologic features of the iPS-RPE were also evaluated.

Results

We obtained iPS-RPE at high purity (approximately 98%). The iPS-RPE showed apical-basal polarity and cellular structure characteristic of RPE. Expression levels of several RPE markers were lower than those of freshly isolated mouse RPE but comparable to those of primary cultured RPE. The iPS-RPE could form tight junctions, phagocytose photoreceptor outer segments, express immune antigens, and suppress lymphocyte proliferation.

Conclusion

We successfully developed a differentiation/purification protocol to obtain mouse iPS-RPE. The mouse iPS-RPE can serve as an attractive tool for functional and morphological studies of RPE.

Analysis of compound heterozygotes reveals that the mouse floxed Pax6 (tm1Ued) allele produces abnormal eye phenotypes

Analysis of abnormal phenotypes produced by different types of mutations has been crucial for our understanding of gene function. Some floxed alleles that retain a neomycin-resistance selection cassette (neo cassette) are not equivalent to wild-type alleles and provide useful experimental resources. Pax6 is an important developmental gene and the aim of this study was to determine whether the floxed Pax6^tm1Ued (Pax6^fl) allele, which has a retained neo cassette, produced any abnormal eye phenotypes that would imply that it differs from the wild-type allele. Homozygous Pax6^fl/fl and heterozygous Pax6^fl/+ mice had no overt qualitative eye abnormalities but morphometric analysis showed that Pax6^fl/fl corneas tended be thicker and smaller in diameter. To aid identification of weak effects, we produced compound heterozygotes with the Pax6^Sey-Neu (Pax6⁻) null allele. Pax6^fl/− compound heterozygotes had more severe eye abnormalities than Pax6^+/− heterozygotes, implying that Pax6^fl differs from the wild-type Pax6⁺ allele. Immunohistochemistry showed that the Pax6^fl/− corneal epithelium was positive for keratin 19 and negative for keratin 12, indicating that it was abnormally differentiated. This Pax6^fl allele provides a useful addition to the existing Pax6 allelic series and this study demonstrates the utility of using compound heterozygotes with null alleles to unmask cryptic effects of floxed alleles.

PAX6: 25th anniversary and more to learn

The DNA-binding transcription factor PAX6 was cloned 25 years ago by multiple teams pursuing identification of human and mouse eye disease causing genes, cloning vertebrate homologues of pattern-forming regulatory genes identified in Drosophila, or abundant eye-specific transcripts. Since its discovery in 1991, genetic, cellular, molecular and evolutionary studies on Pax6 mushroomed in the mid 1990s leading to the transformative thinking regarding the genetic program orchestrating both early and late stages of eye morphogenesis as well as the origin and evolution of diverse visual systems. Since Pax6 is also expressed outside of the eye, namely in the central nervous system and pancreas, a number of important insights into the development and function of these organs have been amassed. In most recent years, genome-wide technologies utilizing massively parallel DNA sequencing have begun to provide unbiased insights into the regulatory hierarchies of specification, determination and differentiation of ocular cells and neurogenesis in general. This review is focused on major advancements in studies on mammalian eye development driven by studies of Pax6 genes in model organisms and future challenges to harness the technology-driven opportunities to reconstruct, step-by-step, the transition from naïve ectoderm, neuroepithelium and periocular mesenchyme/neural crest cells into the three-dimensional architecture of the eye.

Molecular biology and genetics of embryonic eyelid development

The embryology of the eyelid is a complex process that includes interactions between the surface ectoderm and mesenchymal tissues. In the mouse and human, the eyelids form and fuse before birth; they open prenatally in the human and postnatally in the mouse. In the mouse, cell migration is stimulated by different growth factors such as FGF10, TGF-α, Activin B, and HB-EGF. These growth factors modulate downstream BMP4 signaling, the ERK cascade, and JNK/c-JUN. Several mechanisms, such as the Wnt/β-catenin signaling pathway, may inhibit and regulate eyelid fusion. Eyelid opening, on the other hand, is driven by the BMP/Smad signaling system. Several human genetic disorders result from dysregulation of the above molecular pathways.

Lmo4 and Other LIM domain only factors are necessary and sufficient for multiple retinal cell type development

Understanding the molecular basis by which distinct cell types are specified is a central issue in retinogenesis and retinal disease development. Here we examined the role of LIM domain only 4 (Lmo4) in retinal development using both gain-of-function and loss-of-function approaches. By immunostaining, Lmo4 was found to be expressed in mouse retina from E10.5 to mature stages. Retroviral delivery of Lmo4 into retinal progenitor cells could promote the amacrine, bipolar and Müller cell fates at the expense of photoreceptors. It also inhibited the fate of early-born retinal ganglion cells. Using a dominant-negative form of Lmo4 which suppresses transcriptional activities of all LIM domain only factors, we demonstrated that LIM domain only factors are both necessary and sufficient for promoting amacrine and bipolar cell development, but not for the differentiation of ganglion, horizontal, Müller, or photoreceptor cells. Taken together, our study uncovers multiple roles of Lmo4 during retinal development and demonstrates the importance of LIM domain only factors in ensuring proper retinal cell specification and differentiation. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 900-915, 2016.

Neural retina identity is specified by lens-derived BMP signals

The eye has served as a classical model to study cell specification and tissue induction for over a century. Nevertheless, the molecular mechanisms that regulate the induction and maintenance of eye-field cells, and the specification of neural retina cells are poorly understood. Moreover, within the developing anterior forebrain, how prospective eye and telencephalic cells are differentially specified is not well defined. In the present study, we have analyzed these issues by manipulating signaling pathways in intact chick embryo and explant assays. Our results provide evidence that at blastula stages, BMP signals inhibit the acquisition of eye-field character, but from neural tube/optic vesicle stages, BMP signals from the lens are crucial for the maintenance of eye-field character, inhibition of dorsal telencephalic cell identity and specification of neural retina cells. Subsequently, our results provide evidence that a Rax2-positive eye-field state is not sufficient for the progress to a neural retina identity, but requires BMP signals. In addition, our results argue against any essential role of Wnt or FGF signals during the specification of neural retina cells, but provide evidence that Wnt signals together with BMP activity are sufficient to induce cells of retinal pigment epithelial character. We conclude that BMP activity emanating from the lens ectoderm maintains eye-field identity, inhibits telencephalic character and induces neural retina cells. Our findings link the requirement of the lens ectoderm for neural retina specification with the molecular mechanism by which cells in the forebrain become specified as neural retina by BMP activity.

SUMMARY: BMP signals from the lens are crucial to maintain eye-field character, inhibit dorsal telencephalic cell identity, and specificy neural retina cells in chick embryos.

Proanthocyanidins Prevent High Glucose-Induced Eye Malformation by Restoring Pax6 Expression in Chick Embryo

Gestational diabetes mellitus (GDM) is one of the leading causes of offspring malformations, in which eye malformation is an important disease. It has raised demand for therapy to improve fetal outcomes. In this study, we used chick embryo to establish a GDM model to study the protective effects of proanthocyanidins on eye development. Chick embryos were exposed to high glucose (0.2 mmol/egg) on embryo development day (EDD) 1. Proanthocyanidins (1 and 10 nmol/egg) were injected into the air sac on EDD 0. Results showed that both dosages of proanthocyanidins could prevent the eye malformation and rescue the high glucose-induced oxidative stress significantly, which the similar effects were showed in edaravone. However, proanthocyanidins could not decrease the glucose concentration of embryo eye. Moreover, the key genes regulating eye development, Pax6, was down-regulated by high glucose. Proanthocyanidins could restore the suppressed expression of Pax6. These results indicated proanthocyanidins might be a promising natural agent to prevent high glucose-induced eye malformation by restoring Pax6 expression.

Human Ocular Epithelial Cells Endogenously Expressing SOX2 and OCT4 Yield High Efficiency of Pluripotency Reprogramming

A variety of pluripotency reprogramming frequencies from different somatic cells has been observed, indicating cell origin is a critical contributor for efficiency of pluripotency reprogramming. Identifying the cell sources for efficient induced pluripotent stem cells (iPSCs) generation, and defining its advantages or disadvantages on reprogramming, is therefore important. Human ocular tissue-derived conjunctival epithelial cells (OECs) exhibited endogenous expression of reprogramming factors OCT4A (the specific OCT 4 isoform on pluripotency reprogramming) and SOX2. We therefore determined whether OECs could be used for high efficiency of iPSCs generation. We compared the endogenous expression levels of four pluripotency factors and the pluripotency reprograming efficiency of human OECs with that of ocular stromal cells (OSCs). Real-time PCR, microarray analysis, Western blotting and immunostaining assays were employed to compare OECiPSCs with OSCiPSCs on molecular bases of reprogramming efficiency and preferred lineage-differentiation potential. Using the traditional KMOS (KLF4, C-MYC, OCT4 and SOX2) reprogramming protocol, we confirmed that OECs, endogenously expressing reprogramming factors OCT4A and SOX2, yield very high efficiency of iPSCs generation (~1.5%). Furthermore, higher efficiency of retinal pigmented epithelial differentiation (RPE cells) was observed in OECiPSCs compared to OSCiPSCs or skin fibroblast iMR90iPSCs. The findings in this study suggest that conjunctival-derived epithelial (OECs) cells can be easier converted to iPSCs than conjunctival-derived stromal cells (OSCs). This cell type may also have advantages in retinal pigmented epithelial differentiation.

Fibroblast growth factor receptor 2 (FGFR2) is required for corneal epithelial cell proliferation and differentiation during embryonic development

Fibroblast growth factors (FGFs) play important roles in many aspects of embryonic development. During eye development, the lens and corneal epithelium are derived from the same surface ectodermal tissue. FGF receptor (FGFR)-signaling is essential for lens cell differentiation and survival, but its role in corneal development has not been fully investigated. In this study, we examined the corneal defects in Fgfr2 conditional knockout mice in which Cre expression is activated at lens induction stage by Pax6 P0 promoter. The cornea in LeCre, Fgfr2^loxP/loxP mice (referred as Fgfr2^CKO) was analyzed to assess changes in cell proliferation, differentiation and survival. We found that Fgfr2^CKO cornea was much thinner in epithelial and stromal layer when compared to WT cornea. At embryonic day 12.5–13.5 (E12.5–13.5) shortly after the lens vesicle detaches from the overlying surface ectoderm, cell proliferation (judged by labeling indices of Ki-67, BrdU and phospho-histone H3) was significantly reduced in corneal epithelium in Fgfr2^CKO mice. At later stage, cell differentiation markers for corneal epithelium and underlying stromal mesenchyme, keratin-12 and keratocan respectively, were not expressed in Fgfr2^CKO cornea. Furthermore, Pax6, a transcription factor essential for eye development, was not present in the Fgfr2^CKO mutant corneal epithelial at E16.5 but was expressed normally at E12.5, suggesting that FGFR2-signaling is required for maintaining Pax6 expression in this tissue. Interestingly, the role of FGFR2 in corneal epithelial development is independent of ERK1/2-signaling. In contrast to the lens, FGFR2 is not required for cell survival in cornea. This study demonstrates for the first time that FGFR2 plays an essential role in controlling cell proliferation and differentiation, and maintaining Pax6 levels in corneal epithelium via ERK-independent pathways during embryonic development.

Transcriptional regulation of cranial sensory placode development

Cranial sensory placodes derive from discrete patches of the head ectoderm and give rise to numerous sensory structures. During gastrulation, a specialized "neural border zone" forms around the neural plate in response to interactions between the neural and nonneural ectoderm and signals from adjacent mesodermal and/or endodermal tissues. This zone subsequently gives rise to two distinct precursor populations of the peripheral nervous system: the neural crest and the preplacodal ectoderm (PPE). The PPE is a common field from which all cranial sensory placodes arise (adenohypophyseal, olfactory, lens, trigeminal, epibranchial, otic). Members of the Six family of transcription factors are major regulators of PPE specification, in partnership with cofactor proteins such as Eya. Six gene activity also maintains tissue boundaries between the PPE, neural crest, and epidermis by repressing genes that specify the fates of those adjacent ectodermally derived domains. As the embryo acquires anterior-posterior identity, the PPE becomes transcriptionally regionalized, and it subsequently becomes subdivided into specific placodes with distinct developmental fates in response to signaling from adjacent tissues. Each placode is characterized by a unique transcriptional program that leads to the differentiation of highly specialized cells, such as neurosecretory cells, sensory receptor cells, chemosensory neurons, peripheral glia, and supporting cells. In this review, we summarize the transcriptional and signaling factors that regulate key steps of placode development, influence subsequent sensory neuron specification, and discuss what is known about mutations in some of the essential PPE genes that underlie human congenital syndromes.

Pax6- and Six3-mediated induction of lens cell fate in mouse and human ES cells

Embryonic stem (ES) cells provide a potentially useful in vitro model for the study of in vivo tissue differentiation. We used mouse and human ES cells to investigate whether the lens regulatory genes Pax6 and Six3 could induce lens cell fate in vitro. To help assess the onset of lens differentiation, we derived a new mES cell line (Pax6-GFP mES) that expresses a GFP reporter under the control of the Pax6 P0 promoter and lens ectoderm enhancer. Pax6 or Six3 expression vectors were introduced into mES or hES cells by transfection or lentiviral infection and the differentiating ES cells analyzed for lens marker expression. Transfection of mES cells with Pax6 or Six3 but not with other genes induced the expression of lens cell markers and up-regulated GFP reporter expression in Pax6-GFP mES cells by 3 days post-transfection. By 7 days post-transfection, mES cell cultures exhibited a>10-fold increase over controls in the number of colonies expressing γA-crystallin, a lens fiber cell differentiation marker. RT-PCR and immunostaining revealed induction of additional lens epithelial or fiber cell differentiation markers including Foxe3, Prox1, α- and β-crystallins, and Tdrd7. Moreover, γA-crystallin- or Prox1-expressing lentoid bodies formed by 30 days in culture. In hES cells, Pax6 or Six3 lentiviral vectors also induced lens marker expression. mES cells that express lens markers reside close to but are distinct from the Pax6 or Six3 transduced cells, suggesting that the latter induce nearby undifferentiated ES cells to adopt a lens fate by non-cell autonomous mechanisms. In sum, we describe a novel mES cell GFP reporter line that is useful for monitoring induction of lens fate, and demonstrate that Pax6 or Six3 is sufficient to induce ES cells to adopt a lens fate, potentially via non-cell autonomous mechanisms. These findings should facilitate investigations of lens development.

Xenopus laevis FGF receptor substrate 3 (XFrs3) is important for eye development and mediates Pax6 expression in lens placode through its Shp2-binding sites

Members of the fibroblast growth factor (FGF) family play important roles during various developmental processes including eye development. FRS (FGF receptor substrate) proteins bind to FGFR and serve as adapters for coordinated assembly of multi-protein complexes involved in Ras/MAPK and PI3 kinase/Akt pathways. Here, we identified Xenopus laevis Frs3 (XFrs3), a homolog of vertebrate Frs3, and investigated its roles during embryogenesis. XFrs3 is expressed maternally and zygotically with specific expression patterns throughout the early development. Knockdown of XFrs3 using a specific antisense morpholino oligonucleotide (MO) caused reduction of Pax6 expression in the lens placode, and defects in the eye ranging from microphthalmia to anophthalmia. XFrs3 MO-induced defects were alleviated by wild type XFrs3 or a mutant XFrs3 (XFrs3-4YF), in which the putative tyrosine phosphorylation sites served as Grb2-binding sites are mutated. However, another XFrs3 mutant (XFrs3-2YF), in which the putative Shp2-binding sites are mutated, could not rescue the defects of XFrs3 morphants. In addition, we found that XFrs3 is important for FGF or IGF-induced ERK activation in ectodermal tissue. Taken together, our results suggest that signaling through Shp2-binding sites of XFrs3 is necessary for the eye development in Xenopus laevis.

Genetic regulation of vertebrate eye development

Eye development is a complex and highly regulated process that consists of several overlapping stages: (i) specification then splitting of the eye field from the developing forebrain; (ii) genesis and patterning of the optic vesicle; (iii) regionalization of the optic cup into neural retina and retina pigment epithelium; and (iv) specification and differentiation of all seven retinal cell types that develop from a pool of retinal progenitor cells in a precise temporal and spatial manner: retinal ganglion cells, horizontal cells, cone photoreceptors, amacrine cells, bipolar cells, rod photoreceptors and Müller glia. Genetic regulation of the stages of eye development includes both extrinsic (such as morphogens, growth factors) and intrinsic factors (primarily transcription factors of the homeobox and basic helix-loop helix families). In the following review, we will provide an overview of the stages of eye development highlighting the role of several important transcription factors in both normal developmental processes and in inherited human eye diseases.

Pax6 regulates the expression of Dkk3 in murine and human cell lines, and altered responses to Wnt signaling are shown in FlpIn-3T3 cells stably expressing either the Pax6 or the Pax6(5a) isoform

Pax6 is a transcription factor important for early embryo development. It is expressed in several cancer cell lines and tumors. In glioblastoma, PAX6 has been shown to function as a tumor suppressor. Dickkopf 3 (Dkk3) is well established as a tumor suppressor in several tumor types, but not much is known about the regulation of its expression. We have previously found that Pax6 and Pax6(5a) increase the expression of the Dkk3 gene in two stably transfected mouse fibroblast cell lines. In this study the molecular mechanism behind this regulation is looked at. Western blot and reverse transcriptase quantitative PCR (RT-qPCR) confirmed higher level of Dkk3 expression in both Pax6 and Pax6(5a) expressing cell lines compared to the control cell line. By the use of bioinformatics and electrophoretic mobility shift assay (EMSA) we have mapped a functional Pax6 binding site in the mouse Dkk3 promoter. The minimal Dkk3 promoter fragment required for transcriptional activation by Pax6 and Pax6(5a) was a 200 bp region just upstream of the transcriptional start site. Mutation of the evolutionary conserved binding site in this region abrogated transcriptional activation and binding of Pax6/Pax6(5a) to the mouse Dkk3 promoter. Since the identified Pax6 binding site in this promoter is conserved, RT-qPCR and Western blot were used to look for regulation of Dkk3/REIC expression in human cell lines. Six of eight cell lines tested showed changes in Dkk3/REIC expression after PAX6 siRNA knockdown. Interestingly, we observed that the Pax6/Pax6(5a) expressing mouse fibroblast cell lines were less responsive to canonical Wnt pathway stimulation than the control cell line when TOP/FOP activity and the levels of active β-catenin and GSK3-β Ser9 phosphorylation were measured after LiCl stimulation.

The role of homeobox genes in retinal development and disease

Homeobox genes are an evolutionarily conserved class of transcription factors that are critical for development of many organ systems, including the brain and eye. During retinogenesis, homeodomain-containing transcription factors, which are encoded by homeobox genes, play essential roles in the regionalization and patterning of the optic neuroepithelium, specification of retinal progenitors and differentiation of all seven of the retinal cell classes that derive from a common progenitor. Homeodomain transcription factors control retinal cell fate by regulating the expression of target genes required for retinal progenitor cell fate decisions and for terminal differentiation of specific retinal cell types. The essential role of homeobox genes during retinal development is demonstrated by the number of human eye diseases, including colobomas and anophthalmia, which are attributed to homeobox gene mutations. In the following review, we highlight the role of homeodomain transcription factors during retinogenesis and regulation of their gene targets. Understanding the complexities of vertebrate retina development will enhance our ability to drive differentiation of specific retinal cell types towards novel cell-based replacement therapies for retinal degenerative diseases.

Cellular and molecular mechanisms regulating embryonic neurogenesis in the rodent olfactory epithelium

Mechanisms that regulate cellular differentiation in developing embryos are maintained across multiple physiological systems, including the nervous system where neurons and glia are generated. The olfactory epithelium, which arises from the olfactory pit, is a stratified tissue in which the stepwise generation of neurons and support cells can easily be assessed and followed during embryogenesis and throughout adulthood. During olfactory epithelium morphogenesis, progenitor cells respond to factors that control their proliferation, survival, and differentiation in order to generate olfactory receptor neurons that detect odorants in the environment and glia-like sustentacular cells. The tight temporal regulation of expression of proneural genes in dividing progenitor cells, including Mash-1, Neurogenin-1, and NeuroD1, plays a central role in the production of olfactory receptor neurons. Multiple factors that either positively or negatively affect the generation of olfactory receptor neurons have been identified and shown to impinge on the transcriptional regulatory network in dividing progenitor cells. Several growth factors, such as FGF-8, act to promote neurogenesis by ensuring survival of progenitor cells that will give rise to olfactory receptor neurons. In contrast, other molecules, including members of the large TGF-β family of proteins, have negative impacts on neurogenesis by restricting progenitor cell proliferation and stalling their differentiation. Since recent reviews have focused on neurogenesis in the regenerating adult olfactory epithelium, this review describes neurogenesis at embryonic stages of olfactory epithelium development and summarizes our current understanding of how both cell intrinsic and extrinsic factors control this process.

Methods of Retinal Ganglion Cell Differentiation From Pluripotent Stem Cells

Glaucoma, the worldwide leading cause of irreversible blindness, is characterized by progressive degeneration of the optic nerve and loss of retinal ganglion cells. Research into glaucoma pathogenesis has been hampered by difficulties in isolating and culturing retinal ganglion cells in vitro. However, recent improvements in laboratory techniques have enabled the generation of a variety of mature cell types from pluripotent stem cells, including retinal ganglion cells. Indeed, stem cell-based approaches have the potential to revolutionize the field by providing an unlimited source of cells for replacement therapies and by enabling development of in vitro disease models for drug screening and research. Consequently, research aimed at directing pluripotent stem cells to differentiate into retinal ganglion cells has expanded dramatically during the past decade, resulting in significant advances in technique and efficiency. In this paper, we review the methodology for retinal ganglion cell differentiation from pluripotent stem cells of both mouse and human origin and summarize how these techniques have opened up new avenues for modelling glaucoma. Generation of stem cell-derived retinal ganglion cells will have significant translational values, providing an in vitro platform to study the mechanisms responsible for pathogenesis and for drug screening to improve treatment options, as well as for the development of cell therapies for optic neuropathies such as glaucoma.

Suppression of PAX6 promotes cell proliferation and inhibits apoptosis in human retinoblastoma cells

The aim of this study was to investigate the role of the transcription factor, PAX6, in the development of retinoblastoma. The expression of endogenous PAX6 was knocked down using PAX6-specific lentivirus in two human retinoblastoma cell lines, SO-Rb50 and Y79. Cell proliferation functional assays and apoptotic assays were performed on the cells in which PAX6 was knocked down. The results revealed that PAX6 knockdown efficiency was significant (P<0.01, n=3) in the SO-Rb50 and Y79 cells. The inhibition of PAX6 reduced tumor cell apoptosis (P<0.05, n=3), but induced cell cycle S phase arrest (SO-Rb50; P<0.05, n=3) and G2/M phase arrest (Y79; P<0.05, n=3). Western blot analysis indicated that the inhibition of PAX6 increased the levels of the anti-apoptotic proteins, Bcl-2, proliferating cell nuclear antigen (PCNA) and CDK1, but reduced the levels of the pro-apoptotic proteins, BAX and p21. In conclusion, our data demonstrate that the suppression of PAX6 increases proliferation and decreases apoptosis in human retinoblastoma cells by regulating several cell cycle and apoptosis biomarkers.

PAX6 regulates melanogenesis in the retinal pigmented epithelium through feed-forward regulatory interactions with MITF

During organogenesis, PAX6 is required for establishment of various progenitor subtypes within the central nervous system, eye and pancreas. PAX6 expression is maintained in a variety of cell types within each organ, although its role in each lineage and how it acquires cell-specific activity remain elusive. Herein, we aimed to determine the roles and the hierarchical organization of the PAX6-dependent gene regulatory network during the differentiation of the retinal pigmented epithelium (RPE). Somatic mutagenesis of Pax6 in the differentiating RPE revealed that PAX6 functions in a feed-forward regulatory loop with MITF during onset of melanogenesis. PAX6 both controls the expression of an RPE isoform of Mitf and synergizes with MITF to activate expression of genes involved in pigment biogenesis. This study exemplifies how one kernel gene pivotal in organ formation accomplishes a lineage-specific role during terminal differentiation of a single lineage.

It is currently poorly understood how a single developmental transcription regulator controls early specification as well as a broad range of highly specialized differentiation schemes. PAX6 is one of the most extensively investigated factors in central nervous system development, yet its role in execution of lineage-specific programs remains mostly elusive. Here, we directly investigated the involvement of PAX6 in the differentiation of one lineage, the retinal pigmented epithelium (RPE), a neuroectodermal-derived tissue that is essential for retinal development and function. We revealed that PAX6 accomplishes its role through a unique regulatory interaction with the transcription factor MITF, a master regulator of the pigmentation program. During the differentiation of the RPE, PAX6 regulates the expression of an RPE-specific isoform of Mitf and importantly, at the same time, PAX6 functions together with MITF to directly activate the expression of downstream genes required for pigment biogenesis. These findings provide comprehensive insight into the gene hierarchy that controls RPE development: from a kernel gene (a term referring to the upper-most gene in the gene regulatory network) that is broadly expressed during CNS development through a lineage-specific transcription factor that together with the kernel gene creates cis-regulatory input that contributes to transcriptionally activate a battery of terminal differentiation genes.

Distinct Wnt-driven primitive streak-like populations reflect in vivo lineage precursors

During gastrulation, epiblast cells are pluripotent and their fate is thought to be constrained principally by their position. Cell fate is progressively restricted by localised signalling cues from areas including the primitive streak. However, it is unknown whether this restriction accompanies, at the individual cell level, a reduction in potency. Investigation of these early transition events in vitro is possible via the use of epiblast stem cells (EpiSCs), self-renewing pluripotent cell lines equivalent to the postimplantation epiblast. Strikingly, mouse EpiSCs express gastrulation stage regional markers in self-renewing conditions. Here, we examined the differentiation potential of cells expressing such lineage markers. We show that undifferentiated EpiSC cultures contain a major subfraction of cells with reversible early primitive streak characteristics, which is mutually exclusive to a neural-like fraction. Using in vitro differentiation assays and embryo grafting we demonstrate that primitive streak-like EpiSCs are biased towards mesoderm and endoderm fates while retaining pluripotency. The acquisition of primitive streak characteristics by self-renewing EpiSCs is mediated by endogenous Wnt signalling. Elevation of Wnt activity promotes restriction towards primitive streak-associated lineages with mesendodermal and neuromesodermal characteristics. Collectively, our data suggest that EpiSC pluripotency encompasses a range of reversible lineage-biased states reflecting the birth of pioneer lineage precursors from a pool of uncommitted EpiSCs similar to the earliest cell fate restriction events taking place in the gastrula stage epiblast.

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.

The Lhx9 homeobox gene controls pineal gland development and prevents postnatal hydrocephalus

Lhx9 is a member of the LIM homeobox gene family. It is expressed during mammalian embryogenesis in the brain including the pineal gland. Deletion of Lhx9 results in sterility due to failure of gonadal development. The current study was initiated to investigate Lhx9 biology in the pineal gland. Lhx9 is highly expressed in the developing pineal gland of the rat with transcript abundance peaking early in development; transcript levels decrease postnatally to nearly undetectable levels in the adult, a temporal pattern that is generally similar to that reported for Lhx9 expression in other brain regions. Studies with C57BL/6J Lhx9(-/-) mutant mice revealed marked alterations in brain and pineal development. Specifically, the superficial pineal gland is hypoplastic, being reduced to a small cluster of pinealocytes surrounded by meningeal and vascular tissue. The deep pineal gland and the pineal stalk are also reduced in size. Although the brains of neonatal Lhx9(-/-) mutant mice appear normal, severe hydrocephalus develops in about 70% of the Lhx9(-/-) mice at 5-8 weeks of age; these observations are the first to document that deletion of Lhx9 results in hydrocephalus and as such indicate that Lhx9 contributes to the maintenance of normal brain structure. Whereas hydrocephalus is absent in neonatal Lhx9(-/-)mutant mice, the neonatal pineal gland in these animals is hypoplastic. Accordingly, it appears that Lhx9 is essential for early development of the mammalian pineal gland and that this effect is not secondary to hydrocephalus.

Stage-dependent requirement of neuroretinal Pax6 for lens and retina development

The physical contact of optic vesicle with head surface ectoderm is an initial event triggering eye morphogenesis. This interaction leads to lens specification followed by coordinated invagination of the lens placode and optic vesicle, resulting in formation of the lens, retina and retinal pigmented epithelium. Although the role of Pax6 in early lens development has been well documented, its role in optic vesicle neuroepithelium and early retinal progenitors is poorly understood. Here we show that conditional inactivation of Pax6 at distinct time points of mouse neuroretina development has a different impact on early eye morphogenesis. When Pax6 is eliminated in the retina at E10.5 using an mRx-Cre transgene, after a sufficient contact between the optic vesicle and surface ectoderm has occurred, the lens develops normally but the pool of retinal progenitor cells gradually fails to expand. Furthermore, a normal differentiation program is not initiated, leading to almost complete disappearance of the retina after birth. By contrast, when Pax6 was inactivated at the onset of contact between the optic vesicle and surface ectoderm in Pax6(Sey/flox) embryos, expression of lens-specific genes was not initiated and neither the lens nor the retina formed. Our data show that Pax6 in the optic vesicle is important not only for proper retina development, but also for lens formation in a non-cell-autonomous manner.

Necessity of Smad4 for the normal development of the mouse lacrimal gland

PURPOSE

Smad4, a key intracellular mediator in TGF-β signaling, plays a critical role in the normal development of many tissues/organs. However, its functional role in the development of the lacrimal gland (LG) has never been reported. The aim of this study was to investigate the role Smad4 may play in the development of LG by using Smad4 conditional knockout mice and comparing their LG changes with the LG in normal control mice.

METHODS

Smad4 in the LG, as well as in the lens, cornea, and ectoderm of the eyelids, was conditionally inactivated by using Pax6 promoter-driven Cre-transgenic mice. Lac Z reporter was used to visualize the developing LG by X-gal staining, and standard histologic approaches were used to reveal morphologic alterations.

RESULTS

Inactivation of Smad4 resulted in reduction in the size and number of acini in the embryonic LG and in pigment accumulation within the LG, although it did not affect the formation of the primary lacrimal bud. After birth, the LG from Smad4-mutant mice developed slowly, and pigment was observed starting from the P7 mutant LG. Thereafter, the mutant LG was considerably smaller than the normal LG and was eventually replaced by adipose tissue.

CONCLUSIONS

These results support the notion that Smad4 is essential for the normal development and maintenance of the mouse LG.

A survey of ancient conserved non-coding elements in the PAX6 locus reveals a landscape of interdigitated cis-regulatory archipelagos

Biological differences between cell types and developmental processes are characterised by differences in gene expression profiles. Gene-distal enhancers are key components of the regulatory networks that specify the tissue-specific expression patterns driving embryonic development and cell fate decisions, and variations in their sequences are a major contributor to genetic disease and disease susceptibility. Despite advances in the methods for discovery of putative cis-regulatory sequences, characterisation of their spatio-temporal enhancer activities in a mammalian model system remains a major bottle-neck. We employed a strategy that combines gnathostome sequence conservation with transgenic mouse and zebrafish reporter assays to survey the genomic locus of the developmental control gene PAX6 for the presence of novel cis-regulatory elements. Sequence comparison between human and the cartilaginous elephant shark (Callorhinchus milii) revealed several ancient gnathostome conserved non-coding elements (agCNEs) dispersed widely throughout the PAX6 locus, extending the range of the known PAX6 cis-regulatory landscape to contain the full upstream PAX6-RCN1 intergenic region. Our data indicates that ancient conserved regulatory sequences can be tested effectively in transgenic zebrafish even when not conserved in zebrafish themselves. The strategy also allows efficient dissection of compound regulatory regions previously assessed in transgenic mice. Remarkable overlap in expression patterns driven by sets of agCNEs indicates that PAX6 resides in a landscape of multiple tissue-specific regulatory archipelagos.

Developmental, tract-tracing and immunohistochemical study of the peripheral olfactory system in a basal vertebrate: insights on Pax6 neurons migrating along the olfactory nerve

The olfactory system represents an excellent model for studying different aspects of the development of the nervous system ranging from neurogenesis to mechanisms of axon growth and guidance. Important findings in this field come from comparative studies. We have analyzed key events in the development of the olfactory system of the shark Scyliorhinus canicula by combining immunohistochemical and tract-tracing methods. We describe for the first time in a cartilaginous fish an early population of pioneer HuC/D-immunoreactive (ir) neurons that seemed to delaminate from the olfactory pit epithelium and migrate toward the telencephalon before the olfactory nerve was identifiable. A distinct, transient cell population, namely the migratory mass, courses later on in apposition to the developing olfactory nerve. It contains olfactory ensheathing glial (GFAP-ir) cells and HuC/D-ir neurons, some of which course toward an extrabulbar region. We also demonstrate that Pax6-ir cells coursing along the developing olfactory pathways in S. canicula are young migrating (HuC/D and DCX-ir) neurons of the migratory mass that do not form part of the terminal nerve pathway. Evidences that these Pax6 neurons originate in the olfactory epithelium are also reported. As Pax6 neurons in the olfactory epithelium show characteristics of olfactory receptor neurons, and migrating Pax6-ir neurons formed transient corridors along the course of olfactory axons at the entrance of the olfactory bulb, we propose that these neurons could play a role as guideposts for axons of olfactory receptor neurons growing toward the olfactory bulb.