Characterization of Wnt signaling components and activation of the Wnt canonical pathway in the murine retina

Hairy1 acts as a node downstream of Wnt signaling to maintain retinal stem cell-like progenitor cells in the chick ciliary marginal zone

In the vertebrate retina, stem cell-like progenitor cells are maintained in a distinct region called the ciliary marginal zone (CMZ). Canonical Wnt signaling regulates the maintenance of the progenitor cells in the CMZ. However, its downstream molecular mechanisms have remained largely unclear. Here, we show that chick Hairy1, an established Notch signaling effector,mediates the Wnt-dependent maintenance of CMZ progenitor cells in chicken. Interestingly, unlike other developmental contexts in which Hes gene expression is regulated by Notch signaling, Hairy1 expression in the CMZ is regulated by Wnt signaling. Hairy1 is necessary and sufficient for the expression of a set of molecular markers characteristic of the CMZ, and Wnt2b fails to induce CMZ markers when Hairy1 activity is inhibited. Furthermore,microarray analysis identifies multiple Wnt-responsive transcription factors that activate Hairy1 expression. We thus propose that Hairy1 functions as a node downstream of Wnt signaling to maintain progenitor cells in the chick CMZ.

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

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

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

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

Expression of secreted frizzled related protein 1, a Wnt antagonist, in brain, kidney, and skeleton is dispensable for normal embryonic development

Secreted frizzled related protein-1 (sFRP1), an antagonist of Wnt signaling, regulates cell proliferation, differentiation and apoptosis and negatively regulates bone formation. The spatial and temporal pattern of endogenous sFRP1 expression and loss-of-function were examined in the sFRP1-LacZ knock-in mouse (sFRP1-/-) during embryonic development and post-natal growth. beta-gal activity representing sFRP1 expression is robust in brain, skeleton, kidney, eye, spleen, abdomen, heart and somites in early embryos, but sFRP1 gene inactivation in these tissues did not compromise normal embryonic and post-natal development. Kidney histology revealed increased numbers of glomeruli in KO mice, observed after 5 years of breeding. In the skeleton, we show sFRP1 expression is found in relation to the mineralizing front of bone tissue during skeletal development from E15.5 to birth. Trabecular bone volume and bone mineral density in the sFRP1-/- mouse compared to WT was slightly increased during post-natal growth. Calvarial osteoblasts from newborn sFRP1-/- mice exhibited a 20% increase in cell proliferation and differentiation at the early stages of osteoblast maturation. sFRP1 expression was observed in osteoclasts, but this did not affect osteoclast number or activity. These findings have identified functions for sFRP1 in kidney and bone that are not redundant with other sFRPs. In summary, the absence of major organ abnormalities, the enhanced bone formation and a normal life span with no detection of spontaneous tumors suggests that targeting sFRP1 can be used as a therapeutic strategy for increasing bone mass in metabolic bone disorders or promoting fracture healing by modulating Wnt signaling.

Investigation of Frizzled-5 during embryonic neural development in mouse

Recent studies revealed that the Wnt receptor Frizzled-5 (Fzd5) is required for eye and retina development in zebrafish and Xenopus, however, its role during mammalian eye development is unknown. In the mouse embryo, Fzd5 is prominently expressed in the pituitary, distal optic vesicle, and optic stalk, then later in the progenitor zone of the developing retina. To elucidate the role of Fzd5 during eye development, we analyzed embryos with a germline disruption of the Fzd5 gene at E10.25, just before embryos die due to defects in yolk sac angiogenesis. We observed severe defects in optic cup morphogenesis and lens development. However, in embryos with conditional inactivation of Fzd5 using Six3-Cre, we observed no obvious early eye defects. Analysis of Axin2 mRNA expression and TCF/LEF-responsive reporter activation demonstrate that Fzd5 does not regulate the Wnt/beta-catenin pathway in the eye. Thus, the function of Fzd5 during eye development appears to be species-dependent.

Beyond Wnt inhibition: new functions of secreted Frizzled-related proteins in development and disease

The secreted Frizzled-related proteins (SFRPs) are a family of soluble proteins that are structurally related to Frizzled (Fz) proteins, the serpentine receptors that mediate the extensively used cell-cell communication pathway involving Wnt signalling. Because of their homology with the Wnt-binding domain on the Fz receptors, SFRPs were immediately characterised as antagonists that bind to Wnt proteins to prevent signal activation. Since these initial studies, interest in the family of SFRPs has grown progressively, offering new perspectives on their function and mechanism of action in both development and disease. These studies indicate that SFRPs are not merely Wnt-binding proteins, but can also antagonise one another's activity, bind to Fz receptors and influence axon guidance, interfere with BMP signalling by acting as proteinase inhibitors, and interact with other receptors or matrix molecules. Furthermore, their expression is altered in different types of cancers, bone pathologies, retinal degeneration and hypophosphatemic diseases, indicating that their activity is fundamental for tissue homeostasis. Here we review some of the debated aspects of SFRP-Wnt interactions and discuss the new and emerging roles of SFRPs.

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

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

Characterization of a transient TCF/LEF-responsive progenitor population in the embryonic mouse retina

PURPOSE

High mobility group (HMG) transcription factors of the T-cell-specific transcription factor/lymphoid enhancer binding factor (TCF/LEF) family are a class of intrinsic regulators that are dynamically expressed in the embryonic mouse retina. Activation of TCF/LEFs is a hallmark of the Wnt/beta-catenin pathway; however, the requirement for Wnt/beta-catenin and noncanonical Wnt signaling during mammalian retinal development remains unclear. The goal of the study was to characterize more fully a TCF/LEF-responsive retinal progenitor population in the mouse embryo and to correlate this with Wnt/beta-catenin signaling.

METHODS

TCF/LEF activation was analyzed in the TOPgal (TCF optimal promoter) reporter mouse at embryonic ages and compared to Axin2 mRNA expression, an endogenous readout of Wnt/beta-catenin signaling. Reporter expression was also examined in embryos with a retina-specific deletion of the beta-catenin gene (Ctnnb1), using Six3-Cre transgenic mice. Finally, the extent to which TOPgal cells coexpress cell cycle proteins, basic helix-loop-helix (bHLH) transcription factors, and other retinal cell markers was tested by double immunohistochemistry.

RESULTS

TOPgal reporter activation occurred transiently in a subpopulation of embryonic retinal progenitor cells. Axin2 was not expressed in the central retina, and TOPgal reporter expression persisted in the absence of beta-catenin. Although a proportion of TOPgal-labeled cells were proliferative, most coexpressed the cyclin-dependent kinase inhibitor p27/Kip1.

CONCLUSIONS

TOPgal cells give rise to the four earliest cell types: ganglion, amacrine, horizontal, and photoreceptor. TCF/LEF activation in the central retina does not correlate with Wnt/beta-catenin signaling, pointing to an alternate role for this transcription factor family during retinal development.

Canonical Wnt signaling controls proliferation of retinal stem/progenitor cells in postembryonic Xenopus eyes

Vertebrate retinal stem cells, which reside quiescently within the ciliary margin, may offer a possibility for treatment of degenerative retinopathies. The highly proliferative retinal precursor cells in Xenopus eyes are confined to the most peripheral region, called the ciliary marginal zone (CMZ). Although the canonical Wnt pathway has been implicated in the developing retina of different species, little is known about its involvement in postembryonic retinas. Using a green fluorescent protein-based Wnt-responsive reporter, we show that in transgenic Xenopus tadpoles, the canonical Wnt signaling is activated in the postembryonic CMZ. To further investigate the functional implications of this, we generated transgenic, hormone-inducible canonical Wnt pathway activating and repressing systems, which are directed to specifically intersect at the nuclear endpoint of transcriptional Wnt target gene activation. We found that postembryonic induction of the canonical Wnt pathway in transgenic retinas resulted in increased proliferation in the CMZ compartment. This is most likely due to delayed cell cycle exit, as inferred from a pulse-chase experiment on 5-bromo-2'-deoxyuridine-labeled retinal precursors. Conversely, repression of the canonical Wnt pathway inhibited proliferation of CMZ cells. Neither activation nor repression of the Wnt pathway affected the differentiated cells in the central retina. We conclude that even at postembryonic stages, the canonical Wnt signaling pathway continues to have a major function in promoting proliferation and maintaining retinal stem cells. These findings may contribute to the eventual design of vertebrate, stem cell-based retinal therapies. Disclosure of potential conflicts of interest is found at the end of this article.

Indian hedgehog signaling from endothelial cells is required for sclera and retinal pigment epithelium development in the mouse eye

The development of extraocular orbital structures, in particular the choroid and sclera, is regulated by a complex series of interactions between neuroectoderm, neural crest and mesoderm derivatives, although in many instances the signals that mediate these interactions are not known. In this study we have investigated the function of Indian hedgehog (Ihh) in the developing mammalian eye. We show that Ihh is expressed in a population of non-pigmented cells located in the developing choroid adjacent to the RPE. The analysis of Hh mutant mice demonstrates that the RPE and developing scleral mesenchyme are direct targets of Ihh signaling and that Ihh is required for the normal pigmentation pattern of the RPE and the condensation of mesenchymal cells to form the sclera. Our findings also indicate that Ihh signals indirectly to promote proliferation and photoreceptor specification in the neural retina. This study identifies Ihh as a novel choroid-derived signal that regulates RPE, sclera and neural retina development.

The canonical Wnt signaling antagonist DKK2 is an essential effector of PITX2 function during normal eye development

Local control of cell signaling activity and integration of inputs from multiple signaling pathways are central for normal development but the underlying mechanisms remain poorly understood. Here we show that Dkk2, encoding an antagonist of canonical Wnt signaling, is an essential downstream target of the PITX2 homeodomain transcription factor in neural crest during eye development. Canonical Wnt signaling is ectopically activated in central ocular surface ectoderm and underlying mesenchyme in Pitx2- and Dkk2-deficient mice. General ocular surface ectoderm identity is maintained during development in Dkk2-deficient mice but peripheral fates, including conjunctival goblet cells and eyelash follicles, are ectopically permitted within more central structures and eyelids are hypomorphic. Loss of DKK2 results in ectopic blood vessels within the periocular mesenchyme and PITX2 expression remains persistently high, providing evidence for a negative feedback loop. Collectively, these data suggest that activation of Dkk2 by PITX2 provides a mechanism to locally suppress canonical Wnt signaling activity during eye development, a paradigm that may be a model for achieving local or transient inhibition of pathway activity elsewhere during embryogenesis. We further propose a model placing PITX2 as an essential integration node between retinoic acid and canonical Wnt signaling during eye development.

Wnt signaling in retinal stem cells and regeneration

In the vertebrate retina, stem cells with prolonged proliferative capacities reside in the most peripheral region, the ciliary marginal zone (CMZ), and they persist even after the functional eye has formed. These stem cells contribute to the formation of the retinal structures during the postnatal period in vivo, or can expand as neurospheres in vitro. Despite the wealth of anatomical descriptions of the characteristics of CMZ cells, molecular mechanisms for their specification or maintenance have long been uncharacterized. Recent studies provide evidence that certain secreted signaling molecules act as key regulators at multiple steps during these processes. In this review, we discuss the molecular basis for the regulation of retinal stem cells and their related cell types, especially focusing on the role of Wnt signaling.

Iris development in vertebrates; genetic and molecular considerations

The iris plays a key role in visual function. It regulates the amount of light entering the eye and falling on the retina and also operates in focal adjustment of closer objects. The iris is involved in circulation of the aqueous humor and hence functions in regulation of intraocular pressure. Intriguingly, iris pigmented cells possess the ability to transdifferentiate into different ocular cell types of retinal pigmented epithelium, photoreceptors and lens cells. Thus, the iris is considered a potential source for cell-replacement therapies. During embryogenesis, the iris arises from both the optic cup and the periocular mesenchyme. Its interesting mode of development includes specification of the peripheral optic cup to a non-neuronal fate, migration of cells from the surrounding periocular mesenchyme and an atypical formation of smooth muscles from the neuroectoderm. This manner of development raises some interesting general topics concerning the early patterning of the neuroectoderm, the specification and differentiation of diverse cell types and the interactions between intrinsic and extrinsic factors in the process of organogenesis. In this review, we discuss iris anatomy and development, describe major pathologies of the iris and their molecular etiology and finally summarize the recent findings on genes and signaling pathways that are involved in iris development.

Characterization of Wnt signaling during photoreceptor degeneration

PURPOSE

The Wnt pathway is an essential signaling cascade that regulates multiple processes in developing and adult tissues, including differentiation, cellular survival, and stem cell proliferation. The authors recently demonstrated altered expression of Wnt pathway genes during photoreceptor death in rd1 mice, suggesting an involvement for Wnt signaling in the disease process. In this study, the authors investigated the role of Wnt signaling in retinal degeneration.

METHODS

The Wnt signaling reporter mouse line Tcf-LacZ was crossed with retinal degeneration rd1 mice, and beta-galactosidase expression was used to localize Wnt signaling during photoreceptor death. To analyze the role of Wnt signaling activation, primary mixed retinal cultures were prepared, and XTT and TUNEL assays were used to quantify cell death. Luciferase reporter assays were used to measure Wnt signaling.

RESULTS

The canonical Wnt signaling pathway was activated in Müller glia and the ganglion cell layer during rod photoreceptor degeneration in rd1/Tcf-LacZ mice. Wnt signaling was confirmed in cultured primary Müller glia. Furthermore, Wnt signaling activators protected photoreceptors in primary retinal cultures from H(2)O(2)-induced oxidative stress. The Wnt ligands Wnt5a, Wnt5b, Wnt10a, and Wnt13 were expressed in the degenerating retina and are candidate Wnt signaling activators in vivo.

CONCLUSIONS

This study is the first demonstration that Wnt signaling is activated in the degenerating retina and that it protects retinal cultures from oxidative stress. These data suggest that Wnt signaling is a component of the glial protective response during photoreceptor injury. Therefore, inducing Wnt activation, alone or in combination with growth factors, may increase the threshold for apoptosis and halt or delay further photoreceptor degeneration.

Interkinetic nuclear migration and the selection of neurogenic cell divisions during vertebrate retinogenesis

During retinal development, neuroepithelial progenitor cells divide in either a symmetric proliferative mode, in which both daughter cells remain mitotic, or in a neurogenic mode, in which at least one daughter cell exits the cell cycle and differentiates as a neuron. Although the cellular mechanisms of neurogenesis remain unknown, heterogeneity in cell behaviors has been postulated to influence this cell fate. In this study, we analyze interkinetic nuclear migration, the apical-basal movement of nuclei in phase with the cell cycle, and the relationship of this cell behavior to neurogenesis. Using time-lapse imaging in zebrafish, we show that various parameters of interkinetic nuclear migration are significantly heterogeneous among retinal neuroepithelial cells. We provide direct evidence that neurogenic progenitors have greater basal nuclei migrations during the last cell cycle preceding a terminal mitosis. In addition, we show that atypical protein kinase C (aPKC)-mediated cell polarity is essential for the relationship between nuclear position and neurogenesis. Loss of aPKC also resulted in increased proliferative cell divisions and reduced retinal neurogenesis. Our data support a novel model for neurogenesis, in which interkinetic nuclear migration differentially positions nuclei in neuroepithelial cells and therefore influences selection of progenitors for cell cycle exit based on apical-basal polarized signals.

Wnt signaling mediates regional specification in the vertebrate face

At early stages of development, the faces of vertebrate embryos look remarkably similar, yet within a very short timeframe they adopt species-specific facial characteristics. What are the mechanisms underlying this regional specification of the vertebrate face? Using transgenic Wnt reporter embryos we found a highly conserved pattern of Wnt responsiveness in the developing mouse face that later corresponded to derivatives of the frontonasal and maxillary prominences. We explored the consequences of disrupting Wnt signaling, first using a genetic approach. Mice carrying compound null mutations in the nuclear mediators Lef1 and Tcf4 exhibited radically altered facial features that culminated in a hyperteloric appearance and a foreshortened midface. We also used a biochemical approach to perturb Wnt signaling and found that in utero delivery of a Wnt antagonist, Dkk1,produced similar midfacial malformations. We tested the hypothesis that Wnt signaling is an evolutionarily conserved mechanism controlling facial morphogenesis by determining the pattern of Wnt responsiveness in avian faces,and then by evaluating the consequences of Wnt inhibition in the chick face. Collectively, these data elucidate a new role for Wnt signaling in regional specification of the vertebrate face, and suggest possible mechanisms whereby species-specific facial features are generated.

Fibroblast growth factor-hedgehog interdependence during retina regeneration

The embryonic chick is able to regenerate the retina after it has been removed. We have previously shown that proliferating stem/progenitor cells present in the ciliary body/ciliary marginal zone (CB/CMZ) of the chick eye are responsible for regeneration, which can be induced by ectopic fibroblast growth factor-2 (FGF2) or Sonic hedgehog (Shh). Here, we reveal the mechanisms showing how FGF2 and Shh signaling are interdependent during retina regeneration. If the FGF pathway is inhibited, regeneration stimulated by Shh is inhibited. Likewise, if the Hedgehog pathway is inhibited, regeneration stimulated by FGF2 is inhibited. We examined early signaling events in the CB/CMZ and found that FGF2 or Shh induced a robust Erk phosphorylation during the early stages of retina regeneration. Shh also up-regulated the expression of several members of the FGF signaling pathway. We show that ectopic FGF2 or Shh overexpression increased the number of phosphohistone 3 (PH3)-positive cells in the CB/CMZ and inhibition of either pathway decreased the number of PH3-positive cells. Additionally, both FGF and Hh signaling are required for cell survival in the CB/CMZ, whereas Hh and not FGF signaling plays a role in maintaining the identity of the retinal progenitor population in this region. Combined, our results support a model where the FGF and Hedgehog pathways work together to stimulate retina regeneration.

Ciliary margin transdifferentiation from neural retina is controlled by canonical Wnt signaling

The epithelial layers of the ciliary body (CB) and iris are non-neural structures that differentiate from the anterior region of the eyecup, the ciliary margin (CM). We show here that activation of the canonical Wnt signaling pathway is sufficient and necessary for the normal development of anterior eye structures. Pharmacological activation of beta-catenin signaling with lithium (Li(+)) treatment in retinal explants in vitro induced the ectopic expression of the CM markers Otx1 and Msx1. Cre-mediated stabilization of beta-catenin expression in the peripheral retina in vivo induced a cell autonomous upregulation of CM markers at the expense of neural retina (NR) markers and inhibited neurogenesis. Consistent with a cell autonomous conversion to peripheral eye fates, the proliferation index in the region of the retina that expressed stabilized beta-catenin was identical to the wild-type CM and there was an expansion of CB-like structures at later stages. Conversely, Cre-mediated inactivation of beta-catenin reduced CM marker expression as well as the size of the CM and CB/iris. Aberrant CB development in both mouse models was also associated with a reduction in the number of retinal stem cells in vitro. In summary, activation of canonical Wnt signaling is sufficient to promote the development of peripheral eyecup fates at the expense of the NR and is also required for the normal development of anterior eyecup structures.

pygopus 2 has a crucial, Wnt pathway-independent function in lens induction

Drosophila Pygopus was originally identified as a core component of the canonical Wnt signaling pathway and a transcriptional coactivator. Here we have investigated the microophthalmia that arises in mice with a germline null mutation of pygopus 2. We show that this phenotype is a consequence of defective lens development at inductive stages. Using a series of regionally limited Cre recombinase transgenes for conditional deletion of Pygo2flox, we show that Pygo2 activity in pre-placodal presumptive lens ectoderm, placodal ectoderm and ocular mesenchyme all contribute to lens development. In each case, Pygo2 is required for normal expression levels of the crucial transcription factor Pax6. Finally, we provide multiple lines of evidence that although Pygo2 can function in the Wnt pathway, its activity in lens development is Wnt pathway-independent.

Mouse Rab11-FIP4 regulates proliferation and differentiation of retinal progenitors in a Rab11-independent manner

We identified Rab11-family interacting protein 4 (Rab11-FIP4) as a gene strongly expressed in the developing mouse retina. The major transcript encoding a full-length protein, mRab11-FIP4A, was expressed predominantly in neural tissues; whereas an alternative transcript encoding an N-terminally truncated form of the protein, mRab11-FIP4B, was expressed ubiquitously as a minor form. Gain-of-function of mRab11-FIP4A in retina promoted cell cycle exit and increased subpopulations of retinal cells localized in the inner nuclear layer, such as bipolar cells and Müller glia. Reversal of the phenotype was observed in the loss-of-function experiment. Furthermore, Shh signaling was suggested to be involved in these functions. Analysis using truncation mutants revealed the essential role of the N-terminal region containing a conserved EF-hand motif for the retinal phenotypes induced by the expression of mRab11-FIP4A, whereas binding to Rab11 was dispensable, suggesting the involvement of a novel Rab11-independent mechanism for mRab11-FIP4A action in the regulation of retinal development.

Wnt signaling promotes regeneration in the retina of adult mammals

Regeneration in the mammalian CNS is severely limited. Unlike in the chick, current models hold that retinal neurons are never regenerated. Previously we demonstrated that, in the adult mammalian retina, Müller glia dedifferentiate and produce retinal cells, including photoreceptors, after acute neurotoxic injury in vivo. However, the number of newly generated retinal neurons is very limited. Here we demonstrate that Wnt (wingless-type MMTV integration site family)/beta-catenin signaling promotes proliferation of Müller glia-derived retinal progenitors and neural regeneration after damage or during degeneration. Wnt3a treatment increases proliferation of dedifferentiated Müller glia >20-fold in the photoreceptor-damaged retina. Supplementation with retinoic acid or valproic acid induces differentiation of these cells primarily into Crx (cone rod homeobox)-positive and rhodopsin-positive photoreceptors. Notably, injury induces nuclear accumulation of beta-catenin, cyclin D1 upregulation, and Wnt/beta-catenin reporter activity. Activation of Wnt signaling by glycogen synthase kinase-3beta inhibitors promotes retinal regeneration, and, conversely, inhibition of the signaling attenuates regeneration. This Wnt3a-mediated regeneration of retinal cells also occurs in rd mice, a model of retinal degeneration. These results provide evidence that Wnt/beta-catenin signaling contributes to CNS regeneration in the adult mammal.

FGF-mediated induction of ciliary body tissue in the chick eye

Upon morphogenesis, the simple neuroepithelium of the optic vesicle gives rise to four basic tissues in the vertebrate optic cup: pigmented epithelium, sensory neural retina, secretory ciliary body and muscular iris. Pigmented epithelium and neural retina are established through interactions with specific environments and signals: periocular mesenchyme/BMP specifies pigmented epithelium and surface ectoderm/FGF specifies neural retina. The anterior portions (iris and ciliary body) are specified through interactions with lens although the molecular mechanisms of induction have not been deciphered. As lens is a source of FGF, we examined whether this factor was involved in inducing ciliary body. We forced the pigmented epithelium of the embryonic chick eye to express FGF4. Infected cells and their immediate neighbors were transformed into neural retina. At a distance from the FGF signal, the tissue transitioned back into pigmented epithelium. Ciliary body tissue was found in the transitioning zone. The ectopic ciliary body was never in contact with the lens tissue. In order to assess the contribution of the lens on the specification of normal ciliary body, we created optic cups in which the lens had been removed while still pre-lens ectoderm. Ciliary body tissue was identified in the anterior portion of lens-less optic cups. We propose that the ciliary body may be specified at optic vesicle stages, at the same developmental stage when the neural retina and pigmented epithelium are specified and we present a model as to how this could be accomplished through overlapping BMP and FGF signals.

Molecular mechanisms of optic vesicle development: complexities, ambiguities and controversies

Optic vesicle formation, transformation into an optic cup and integration with neighboring tissues are essential for normal eye formation, and involve the coordinated occurrence of complex cellular and molecular events. Perhaps not surprisingly, these complex phenomena have provided fertile ground for controversial and even contradictory results and conclusions. After presenting an overview of current knowledge of optic vesicle development, we will address conceptual and methodological issues that complicate research in this field. This will be done through a review of the pertinent literature, as well as by drawing on our own experience, gathered through recent studies of both intra- and extra-cellular regulation of optic vesicle development and patterning. Finally, and without attempting to be exhaustive, we will point out some important aspects of optic vesicle development that have not yet received enough attention.

c-kit marks late retinal progenitor cells and regulates their differentiation in developing mouse retina

Retinal progenitor cells are believed to display altered proliferation and differentiation during retinal development, suggesting that retinal progenitor cell populations are not homogeneous. However, the composition of progenitor cell populations is not known, due in part to the lack of known surface markers identifying distinct stages of retinal progenitor cells. We found a dramatic change in the expression profile of the cell surface antigens c-kit and stage-specific embryonic antigen-1 (SSEA-1) in retinal progenitor cells during development. While SSEA-1 was expressed early in development, c-kit expression peaked in late stage progenitor cells. The identification of these developmental markers enabled us to characterize distinct sub-populations of retinal progenitor cells. Progenitor cell subpopulations expressing either SSEA-1, c-kit, or both showed different proliferation and differentiation abilities. Although SSEA-1-positive cells were augmented by beta-catenin signaling, c-kit-positive cells were positively regulated by Notch signaling. Taken together, our data suggest that c-kit and SSEA-1 can be used to spatiotemporally differentiate retinal progenitor populations that have intrinsically distinct characteristics. Prolonged expression of c-kit by a retrovirus resulted in the promotion of proliferation and the appearance of nestin-positive cells in the presence of the c-kit ligand, stem cell factor (SCF). This suggests a role for c-kit, Notch, and the beta-catenin signaling network in retinal development.

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

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

A role for Wnt/planar cell polarity signaling during lens fiber cell differentiation?

Wnt signaling through frizzled (Fz) receptors plays key roles in just about every developmental system that has been studied. Several Wnt-Fz signaling pathways have been identified including the Wnt/planar cell polarity (PCP) pathway. PCP signaling is crucial for many developmental processes that require major cytoskeletal rearrangements. Downstream of Fz, PCP signaling is thought to involve the GTPases, Rho, Rac and Cdc42 and regulation of the JNK cascade. Here we report on the localization of these GTPases and JNK in the lens and assess their involvement in the cytoskeletal reorganisation that is a key element of FGF-induced lens fiber cell differentiation.

Modifications of the retina neuronal populations of the heterozygous mutant small eye mouse, the Sey(Dey)

We analyzed the modifications of the retinal neurons in a heterozygous mutant small eye mouse, the Sey(Dey). This mouse presents a mutation in chromosome 2 which affects the gene Pax6 and other nearby genes, such as the Wt1 gene and the gene of the Reticulocalbin. The eyes of these animals do not have lenses and their retinas present important morphological alterations: in the anterior portion they are joined to the cornea, they are found detached from the pigment epithelium, they present folds that form rosettes in some zones and alteration of the lamination can be observed. The partial loss of the genes affected does not prevent the formation of the different layers of the retina, but does affect its thickness, principally of the plexiform layers; moreover, the internal limiting membrane is found disorganized. All the neuronal populations are present in the retina of these animals and express the same neurochemical markers as the control animals, but the number of Pax6(+) cells is notably reduced. In these retinas a marked disorganization of the distribution of the dendrites and axons is observed and a notable reduction in the axons of ganglion cells. These results suggest that, although it does not appear determinant in the differentiation of the distinct neuronal types of the retina, the partial lack of genes of the heterozygotes +/Sey(Dey) provokes important morphological and neurochemical modifications in the cytoarchitecture of the retina.

Determinative role of Wnt signals in dorsal iris-derived lens regeneration in newt eye

We have previously shown that lens regeneration from the pigmented epithelium of the dorsal iris in the adult newt eye proceeds in two steps after lens removal or intraocular FGF2 injection. The FGF2-dependent proliferation of iris pigmented epithelium and activation of early lens genes that occur over the entire circumference of the iris comprise the first step, while subsequent dorsally confined lens development marks the second step. Here, we investigated the expression of Wnt and Wnt receptor Frizzled genes in lens-regenerating iris tissues. Wnt2b and Frizzled4 were activated only in the dorsal half of the iris in synchrony with the occurrence of the second step, whereas Wnt5a and Frizzled2 were activated in both halves throughout the period of the first and second steps. Cultured explants of the iris-derived pigmented epithelium in the presence of FGF2 underwent dorsal-specific lens development fully recapitulating the in vivo lens regeneration process. Under these conditions, Wnt inhibitors Dkk1, which specifically inhibits the canonical signal pathway, and/or sFRP1 repressed the lens development, while exogenous Wnt3a, which generally activates the canonical pathway like Wnt2b, stimulated lens development from the dorsal iris epithelium and even caused lens development from the ventral iris epithelium, albeit at a reduced rate. Wnt5a did not elicit lens development from the ventral epithelium. These observations indicate that dorsal-specific activation of Wnt2b determines the dorsally limited development of lens from the iris pigmented epithelium.

Dual roles for adenomatous polyposis coli in regulating retinoic acid biosynthesis and Wnt during ocular development

Congenital hypertrophy/hyperplasia of the retinal pigmented epithelium is an ocular lesion found in patients harboring mutations in the adenomatous polyposis coli (APC) tumor suppressor gene. We report that Apc-deficient zebrafish display developmental abnormalities of both the lens and retina. Injection of dominant-negative Lef reduced Wnt signaling in the lens but did not rescue retinal differentiation defects. In contrast, treatment of apc mutants with all-trans retinoic acid rescued retinal differentiation defects but had no apparent effect on the lens. We identified Rdh5 as a retina-specific retinol dehydrogenase controlled by APC. Morpholino knockdown of Rdh5 phenocopied the apc mutant retinal differentiation defects and was rescued by treatment with exogenous all-trans retinoic acid. Microarray analyses of apc mutants and Rdh5 morphants revealed a profound overlap in the transcriptional profile of these embryos. These findings support a model wherein Apc serves a dual role in regulating Wnt and retinoic acid signaling within the eye and suggest retinoic acid deficiency as an explanation for APC mutation-associated retinal defects such as congenital hypertrophy/hyperplasia of the retinal pigmented epithelium.

Wnt2b/β-catenin-mediated canonical Wnt signaling determines the peripheral fates of the chick eye

Wnt signaling orchestrates multiple aspects of central nervous system development, including cell proliferation and cell fate choices. In this study, we used gene transfer to activate or inhibit canonical Wnt signaling in vivo in the developing eye. We found that the expression of Wnt2b or constitutively active (CA) β-catenin inhibited retinal progenitor gene(RPG) expression and the differentiation of retinal neurons. In addition, Wnt signal activation in the central retina was sufficient to induce the expression of markers of the ciliary body and iris, two tissues derived from the peripheral optic cup (OC). The expression of a dominant-negative (DN)allele of Lef1, or of a Lef1-engrailed fusion protein, led to the inhibition of expression of peripheral genes and iris hypoplasia, suggesting that canonical Wnt signaling is required for peripheral eye development. We propose that canonical Wnt signaling in the developing optic vesicle (OV) and OC plays a crucial role in determining the identity of the ciliary body and iris. Because wingless (wg) plays a similar role in the induction of peripheral eye tissues of Drosophila, these findings indicate a possible conservation of the process that patterns the photoreceptive and support structures of the eye.

Beta-catenin is essential for lamination but not neurogenesis in mouse retinal development

During vertebrate retinal development, the seven retinal cell types differentiate sequentially from a single population of retinal progenitor cells (RPCs) and organize themselves into a distinct laminar structure. The purpose of this study was to determine whether beta-catenin, which functions both as a nuclear effector for the canonical Wnt signaling pathway and as a regulator of cell adhesion, is required for retinal neurogenesis or lamination. We used the Cre-loxP system to either eliminate beta-catenin or to express a constitutively active form during retinal neurogenesis. Eliminating beta-catenin did not affect cell differentiation, but did result in the loss of the radial arrangement of RPCs and caused abnormal migration of differentiated neurons. As a result, the laminar structure was massively disrupted in beta-catenin-null retinas, although all retinal cell types still formed. In contrast to other neural tissues, eliminating beta-catenin did not significantly reduce the proliferation rate of RPCs; likewise, activating beta-catenin ectopically in RPCs did not result in overproliferation, but loss of neural retinal identity. These results indicate that beta-catenin is essential during retinal neurogenesis as a regulator of cell adhesion but not as a nuclear effector of the canonical Wnt signaling pathway. The results further imply that retinal lamination and retinal cell differentiation are genetically separable processes.

Neural stem cell properties of Müller glia in the mammalian retina: regulation by Notch and Wnt signaling

The retina in adult mammals, unlike those in lower vertebrates such as fish and amphibians, is not known to support neurogenesis. However, when injured, the adult mammalian retina displays neurogenic changes, raising the possibility that neurogenic potential may be evolutionarily conserved and could be exploited for regenerative therapy. Here, we show that Müller cells, when retrospectively enriched from the normal retina, like their radial glial counterparts in the central nervous system (CNS), display cardinal features of neural stem cells (NSCs), i.e., they self-renew and generate all three basic cell types of the CNS. In addition, they possess the potential to generate retinal neurons, both in vitro and in vivo. We also provide direct evidence, by transplanting prospectively enriched injury-activated Müller cells into normal eye, that Müller cells have neurogenic potential and can generate retinal neurons, confirming a hypothesis, first proposed in lower vertebrates. This potential is likely due to the NSC nature of Müller cells that remains dormant under the constraint of non-neurogenic environment of the adult normal retina. Additionally, we demonstrate that the mechanism of activating the dormant stem cell properties in Müller cells involves Wnt and Notch pathways. Together, these results identify Müller cells as latent NSCs in the mammalian retina and hence, may serve as a potential target for cellular manipulation for treating retinal degeneration.

Molecular characterization of retinal stem cells and their niches in adult zebrafish

Background

The persistence in adult teleost fish of retinal stem cells that exhibit all of the features of true 'adult stem cells' – self-renewal, multipotency, and the capacity to respond to injury by mitotic activation with the ability to regenerate differentiated tissues – has been known for several decades. However, the specialized cellular and molecular characteristics of these adult retinal stem cells and the microenvironmental niches that support their maintenance in the differentiated retina and regulate their activity during growth and regeneration have not yet been elucidated.

Results

Our data show that the zebrafish retina has two kinds of specialized niches that sustain retinal stem cells: 1) a neuroepithelial germinal zone at the interface between neural retina and ciliary epithelium, called the ciliary marginal zone (CMZ), a continuous annulus around the retinal circumference, and 2) the microenvironment around some Müller glia in the differentiated retina. In the uninjured retina, scattered Müller glia (more frequently those in peripheral retina) are associated with clusters of proliferating retinal progenitors that are restricted to the rod photoreceptor lineage, but following injury, the Müller-associated retinal progenitors can function as multipotent retinal stem cells to regenerate other types of retinal neurons. The CMZ has several features in common with the neurogenic niches in the adult mammalian brain, including access to the apical epithelial surface and a close association with blood vessels. Müller glia in the teleost retina have a complex response to local injury that includes some features of reactive gliosis (up-regulation of glial fibrillary acidic protein, GFAP, and re-entry into the cell cycle) together with dedifferentiation and re-acquisition of phenotypic and molecular characteristics of multipotent retinal progenitors in the CMZ (diffuse distribution of N-cadherin, activation of Notch-Delta signaling, and expression of rx1, vsx2/Chx10, and pax6a) along with characteristics associated with radial glia (expression of brain lipid binding protein, BLBP). We also describe a novel specific marker for Müller glia, apoE.

Conclusion

The stem cell niches that support multi-lineage retinal progenitors in the intact, growing and regenerating teleost retina have properties characteristic of neuroepithelia and neurogenic radial glia. The regenerative capacity of the adult zebrafish retina with its ability to replace lost retinal neurons provides an opportunity to discover the molecular regulators that lead to functional repair of damaged neural tissue.

Effects of follistatin overexpression on cell differentiation in the chick embryo retina

Although activin is expressed in the embryonic central nervous system (CNS), its possible functions in the regulation of CNS neuronal differentiation remain largely unknown. We have investigated this question in the retina, a well-characterized CNS structure previously shown to respond to activin in vitro, and to express activin subunits and receptors in vivo. RCAS retroviruses were used to overexpress in the chick retina in ovo either follistatin (FS), an activin-binding protein and inhibitor, or alkaline phosphatase (AP), as control. FS-treated retinas appeared normal until ED 8, when they showed a reduction of the inner plexiform layer, accompanied by a marked decrease in the frequency of amacrine cells. The territory lacking amacrine cells showed downregulation of transcription factors necessary for amacrine cell differentiation, such as Pax6 and AP2alpha, accompanied by ectopic expression of transcription factors associated with the development of horizontal or bipolar neurons, such as Prox1, Chx10 and NeuroM. Increases in cell death were also observed in FS-treated retinas. Taken together with previous in vitro studies, our results suggest that activin is a powerful regulator of neuronal differentiation in the central nervous system.

Exogenous FGF10 can rescue an eye-open at birth phenotype of Fgf10-null mice by activating activin and TGFalpha-EGFR signaling

Mutant mice deficient in the fibroblast growth factor 10 (Fgf10) gene exhibit an eye-open phenotype at birth. It has previously been shown that FGF10 has a dual role in proliferation and migration during the early and later stages of eyelid development, respectively. To verify the role of FGF10 during eyelid closure, explant culture of Fgf10-null eyelid anlagen was performed, by which it was examined whether or not exogenous FGF10 could rescue the expression of activin betaB and transforming growth factor alpha, known to be required for eyelid closure. We found that the expression of these genes was markedly induced while that of Shh or Ptch1, Ptch2 was not. We also observed the distribution of filamentous actin (F-actin) after FGF10 application in the mutant eyelid explant, finding that the FGF10 protein induced F-actin accumulation. We further examined filopodia of the eyelid leading edge cells, finding the length of the filopodia was significantly reduced in the mutant. These results verify that FGF10 promotes eyelid closure through activating activin and TGFalpha-EGFR signaling.

Beta-catenin activation is necessary and sufficient to specify the dorsal dermal fate in the mouse

Dorsal dermis and epaxial muscle have been shown to arise from the central dermomyotome in the chick. En1 is a homeobox transcription factor gene expressed in the central dermomyotome. We show by genetic fate mapping in the mouse that En1-expressing cells of the central dermomyotome give rise to dorsal dermis and epaxial muscle and, unexpectedly, to interscapular brown fat. Thus, the En1-expressing central dermomyotome normally gives rise to three distinct fates in mice. Wnt signals are important in early stages of dermomyotome development, but the signal that acts to specify the dermal fate has not been identified. Using a reporter transgene for Wnt signal transduction, we show that the En1-expressing cells directly underneath the surface ectoderm transduce Wnt signals. When the essential Wnt transducer beta-catenin is mutated in En1 cells, it results in the loss of Dermo1-expressing dorsal dermal progenitors and dermis. Conversely, when beta-catenin was activated in En1 cells, it induces Dermo1 expression in all cells of the En1 domain and disrupts muscle gene expression. Our results indicate that the mouse central dermomyotome gives rise to dermis, muscle, and brown fat, and that Wnt signalling normally instructs cells to select the dorsal dermal fate.

SSEA-1 marks regionally restricted immature subpopulations of embryonic retinal progenitor cells that are regulated by the Wnt signaling pathway

Identification and expansion of retinal progenitor cells are critical issues from both scientific and clinical aspects. Here, we identified SSEA-1 (CD15) as a novel surface antigen that can be used to define immature retinal progenitor cells. SSEA-1-expressing retinal cells were found in the peripheral region of the early embryonic mouse retina, and then their number dramatically disappeared along with retinal development. FACS analysis showed that the cells strongly positive for SSEA-1 co-expressed Ki67 proliferation antigen in all the developmental stages examined. The SSEA-1-expressing cells formed larger colonies than the non-expressing ones in retinal re-aggregation cultures. Moreover, late onset of rhodopsin expression was observed in SSEA-1-positive progenitor cells, supporting the idea that these cells have an intrinsically immature character. Differential expression of Wnt signal-related genes between SSEA-1-positive and -negative subpopulations of retina cells was revealed, and the expression of constitutively active forms of Wnt signaling molecules resulted in a greater number of SSEA-1-positive cells. In light of all of the data taken together, we propose SSEA-1 to be a surface marker to define a regionally restricted immature subset of progenitor cells of mouse neural retina, with SSEA-1 expression by them positively regulated by Wnt signals.

Expression patterns of Wnt genes during development of an anterior part of the chicken eye

To address the roles of Wnts in the development of the anterior eye, we used a chicken model to perform comprehensive expression analysis of all Wnt genes during anterior eye development. In analyzing the available genomic sequences, we found that the chicken genome encodes 18 Wnt proteins that are homologous to corresponding human and mouse proteins. The mRNA sequences for 12 chicken Wnt genes are available in GenBank, and mRNAs for six other Wnt genes (Wnt2, Wnt5b, Wnt7b, Wnt8b, Wnt9b, and Wnt16) were identified and cloned based on the homology to the genes from other species. In addition, we found that chicken Wnt3a and Wnt7b genes encode two alternative mRNA isoforms containing different first exons. Following in situ hybridization, we found that out of 18 Wnt genes, 11 genes were expressed in the anterior eye, exhibiting distinct temporal-spatial patterns. Several Wnts were expressed in the lens, including Wnt2 and Wnt2b in the anterior epithelium and Wnt5a, Wnt5b, Wnt7a, and Wnt7b in the differentiating lens fiber cells. In the cornea, we detected Wnt3a, Wnt6, and Wnt9b in the ocular surface ectoderm, including the corneal epithelium, and Wnt9a in the corneal endothelium from the onset of its differentiation. In the optic cup, Wnt2, Wnt2b, and Wnt9a were localized in the rim of the optic cup (presumptive iris), while Wnt5a and Wnt16 were detected in the ciliary epithelium/iris zone of the differentiated optic cup, and Wnt6 was expressed in the iridial mesenchyme. These data suggest that Wnt signaling might play important roles in anterior eye development.

Negative regulation of retinal-neurite extension by beta-catenin signaling pathway

Although there have been many studies on the regulation of neurite extension in mouse brain, such a mechanism in neural retina has remained to be clarified. To delineate the role of Wnt signaling in retinal development, we used a retrovirus-vector-mediated expression system to express various mutants forms of Wnt signaling members in E17.5 mouse retinal explant cultures, which are an excellent system to examine retinal development in vitro. Expression of constitutively active beta-catenin or Lef-1 in the retinal cells resulted in failure of neurite extension, suggesting that beta-catenin negatively regulates neurite extension in the retina through Lef-1 transcriptional activity. However, proliferation and differentiation of retinal cells into mature retinal cells such as rod-photoreceptor cells and Muller glia cells were not affected by perturbation of the Wnt-Lef-1 pathway. As in retinal cells, activation of beta-catenin-Lef-1 signaling inhibited NGF-induced neurite extension in PC12 cells without affecting their proliferation. Interestingly, the Wnt-Lef-1 signaling pathway suppressed neurite extension without affecting Mek-1 signal activity, which is known to promote neurite extension. We found that MAPK was activated in retinal explant cultures, but that perturbation of MAPK signals did not affect neurite extension. Taken together, our data suggest that the Wnt pathway functions in proper neurite extension by opposing positive signals for promotion of neurite extension that are distinct from those of the MAPK pathway.

Mitochondrial and nuclear forms of Wnt13 are generated via alternative promoters, alternative RNA splicing, and alternative translation start sites

Wnt proteins play a key role in cell survival, cell proliferation, and cell fate during development. In endothelial cells, we identified the expression of Wnt13A, Wnt13B, and Wnt13C mRNAs, which are generated by alternative promoters and alternative RNA splicing. Wnt13A and Wnt13B proteins differ only in their N-terminal sequences. Wnt13A, a typical Wnt, is N-glycosylated and localized in the endoplasmic reticulum, with only a small fraction being secreted. Wnt13B proteins appear as a protein doublet, L-Wnt13B and S-Wnt13B, which are neither N-glycosylated nor secreted. Wnt13B proteins localized mainly to mitochondria, as demonstrated using detection in mitochondria enriched fractions and colocalization with Mitotracker and HSP60. A nuclear localization was also observed in 20% of Wnt13B-expressing cells. Both the N-terminal hydrophobic stretch (residues 1-17) and alpha-helix (residues 26-50) were the main determinants for Wnt13B mitochondrial targeting. Serial deletions of Wnt13B N-terminal sequences abolished its association with mitochondria and favored instead a nuclear localization. The production of S-Wnt13B was independent of the mitochondrial targeting but dependent on an alternative translation start corresponding to Met(74) in L-Wnt13B. The same translation start is used in Wnt13C mRNA to encode a protein undistinguishable from S-Wnt13B. S-Wnt13B when expressed alone localized to the nucleus like Wnt13C, whereas L-Wnt13B localized to mitochondria. Wnt13 nuclear forms increased the beta-catenin/T-cell factor activity in HEK293 cells and increased apoptosis in bovine aortic endothelial cells. Altogether our results demonstrate that, in addition to alternative promoters and RNA splicing, an alternative translation start in Wnt13B and Wnt13C mRNAs increases the complexity of both human wnt13 expression and functions.

Neural stem cells in the adult ciliary epithelium express GFAP and are regulated by Wnt signaling

The identification of neural stem cells with retinal potential in the ciliary epithelium (CE) of the adult mammals is of considerable interest because of their potential for replacing or rescuing degenerating retinal neurons in disease or injury. The evaluation of such a potential requires characterization of these cells with regard to their phenotypic properties, potential, and regulatory mechanisms. Here, we demonstrate that rat CE stem cells/progenitors in neurosphere culture display astrocytic nature in terms of expressing glial intermediate neurofilament protein, GFAP. The GFAP-expressing CE stem cells/progenitors form neurospheres in proliferating conditions and generate neurons when shifted to differentiating conditions. These cells express components of the canonical Wnt pathway and its activation promotes their proliferation. Furthermore, we demonstrate that the activation of the canonical Wnt pathway influences neuronal differentiation of CE stem cells/progenitors in a context dependent manner. Our observations suggest that CE stem cells/progenitors share phenotypic properties and regulatory mechanism(s) with neural stem cells elsewhere in the adult CNS.

Membrane properties of retinal stem cells/progenitors

The membrane properties of cells help integrate extrinsic information relayed through growth factors, chemokines, extracellular matrix, gap junctions and neurotransmitters towards modulating cell-intrinsic properties, which in turn determine whether cells remain quiescent, proliferate, differentiate, establish contact with other cells or remove themselves by activating programmed cell death. This review highlights some of the membrane properties of early and late retinal stem cells/progenitors, which are likely to be helpful in the identification and enrichment of these cells and in understanding mechanisms underlying their maintenance and differentiation. Understanding of membrane properties of retinal stem cells/progenitors is essential for the successful formulation of approaches to treat retinal degeneration and diseases by cell therapy.

Activation of canonical Wnt pathway promotes proliferation of retinal stem cells derived from adult mouse ciliary margin

Adult retinal stem cells represent a possible cell source for the treatment of retinal degeneration. However, only a small number of stem cells reside in the ciliary margin. The present study aimed to promote the proliferation of adult retinal stem cells via the Wnt signaling pathway. Ciliary margin cells from 8-week-old mice were dissociated and cultured to allow sphere colony formation. Wnt3a, a glycogen synthase kinase (GSK) 3 inhibitor, fibroblast growth factor (FGF) 2, and a FGF receptor inhibitor were then applied in the culture media. The primary spheres were dissociated to prepare either monolayer or secondary sphere cultures. Wnt3a increased the size of the primary spheres and the number of Ki-67-positive proliferating cells in monolayer culture. The Wnt3a-treated primary sphere cells were capable of self-renewal and gave rise to fourfold the number of secondary spheres compared with nontreated sphere cells. These cells also retained their multilineage potential to express several retinal markers under differentiating culture conditions. The Wnt3a-treated cells showed nuclear accumulation of beta-catenin, and a GSK3 inhibitor, SB216763, mimicked the mitogenic activity of Wnt3a. The proliferative effect of SB216763 was attenuated by an FGF receptor inhibitor but was enhanced by FGF2, with Ki-67-positive cells reaching over 70% of the total cells. Wnt3a and SB216763 promoted the proliferation of retinal stem cells, and this was partly dependent on FGF2 signaling. A combination of Wnt and FGF signaling may provide a therapeutic strategy for in vitro expansion or in vivo activation of adult retinal stem cells.

The Wnt signaling pathway in retinal degenerations

The retina is a complex tissue composed of multiple interconnected cell layers, highly specialized for transforming light and color into electrical signals perceived by the brain. Damage or death of the primary light-sensing cells, the photoreceptors, results in devastating effects on vision. Despite the identification of numerous mutations that cause inherited retinal degenerations, the cellular and molecular mechanisms leading from the primary mutations to photoreceptor apoptosis are not understood. Wnt signaling has essential regulatory functions in a wide variety of critical developmental processes. Our research and others' have suggested that the Wnt pathway may be involved in retinal degeneration. Wnt ligands regulate developmental death of Drosophila photoreceptors, dysregulated Wnt signaling is involved in neuronal degeneration elsewhere in the central nervous system and Wnts control the expression of pro-survival growth factors in mammalian tissues. Additionally, altered expression of Wnt pathway genes, including the anti-apoptotic Wnt signaling regulator Dickkopf 3 (Dkk3), were observed during photoreceptor loss. This review examines the evidence and develops a model proposing a pro-survival role for Wnt signaling during photoreceptor injury. Because manipulating Wnt signaling has been demonstrated to have therapeutic potential for the treatment of Alzheimers disease, understanding the involvement of Wnts in photoreceptor death will determine whether targeting the Wnt pathway should also be considered as a possible therapeutic strategy for retinal degenerations.

SFRP1 regulates the growth of retinal ganglion cell axons through the Fz2 receptor

Axon growth is governed by the ability of growth cones to interpret attractive and repulsive guidance cues. Recent studies have shown that secreted signaling molecules known as morphogens can also act as axon guidance cues. Of the large family of Wnt signaling components, only Wnt4 and Wnt5 seem to participate directly in axon guidance. Here we show that secreted Frizzled-related protein 1 (SFRP1), a proposed Wnt signaling inhibitor, can directly modify and reorient the growth of chick and Xenopus laevis retinal ganglion cell axons. This activity does not require Wnt inhibition and is modulated by extracellular matrix molecules. Intracellularly, SFRP1 function requires G(alpha) protein activation, protein synthesis and degradation, and it is modulated by cyclic nucleotide levels. Because SFRP1 interacts with Frizzled-2 (Fz2) and interference with Fz2 expression abolishes growth cone responses to SFRP1, we propose a previously unknown function for this molecule: the ability to guide growth cone movement via the Fz2 receptor.

Histone deacetylase 1 regulates retinal neurogenesis in zebrafish by suppressing Wnt and Notch signaling pathways

In the developing vertebrate retina, progenitor cells initially proliferate but begin to produce postmitotic neurons when neuronal differentiation occurs. However, the mechanism that determines whether retinal progenitor cells continue to proliferate or exit from the cell cycle and differentiate is largely unknown. Here, we report that histone deacetylase 1 (Hdac1) is required for the switch from proliferation to differentiation in the zebrafish retina. We isolated a zebrafish mutant, ascending and descending (add), in which retinal cells fail to differentiate into neurons and glial cells but instead continue to proliferate. The cloning of the add gene revealed that it encodes Hdac1. Furthermore, the ratio of the number of differentiating cells to that of proliferating cells increases in proportion to Hdac activity, suggesting that Hdac proteins regulate a crucial step of retinal neurogenesis in zebrafish. Canonical Wnt signaling promotes the proliferation of retinal cells in zebrafish, and Notch signaling inhibits neuronal differentiation through the activation of a neurogenic inhibitor, Hairy/Enhancer-of-split (Hes). We found that both the Wnt and Notch/Hes pathways are activated in the add mutant retina. The cell-cycle progression and the upregulation of Hes expression in the add mutant retina can be inhibited by the blockade of Wnt and Notch signaling, respectively. These data suggest that Hdac1 antagonizes these pathways to promote cell-cycle exit and the subsequent neurogenesis in zebrafish retina. Taken together, these data suggest that Hdac1 functions as a dual switch that suppresses both cell-cycle progression and inhibition of neurogenesis in the zebrafish retina.

The duality of β-catenin function: A requirement in lens morphogenesis and signaling suppression of lens fate in periocular ectoderm

In the current analysis, we have investigated both the cytoskeletal and signaling roles of beta-catenin during the early phases of lens development using conditional loss- and gain-of-function strategies. Conditional loss of beta-catenin in the presumptive lens does not perturb the normal sequential appearance of lens fate markers but results in a dramatic failure of the coordinated epithelial cell behavior that constitutes lens morphogenesis. Similarly, loss-of-function for Lrp6, the Wnt pathway coreceptor expressed in the eye primordium, does not prevent expression of lens induction markers. Surprisingly, conditional deletion of beta-catenin in periocular ectoderm results in the formation of Prox-1 and beta-crystallin-positive ectopic lentoid bodies. Combined with the observation that the Wnt pathway reporter TOPGAL is expressed in nasal periocular ectoderm, these data suggest that, in this location, the canonical Wnt signaling pathway normally suppresses lens fate in favor of other structures. Consistent with this proposal, a dominant-active form of beta-catenin causes a loss of lens fate and a complete absence of lens development when expressed in the presumptive lens ectoderm.